Remove obsolete usages of @OptIn(ExperimentalSerializationApi::class)

They were connected with SerialDescriptor, which is @SubclassOptInRequired since 1.8.0

Author: sandwwraith

core/commonMain/src/kotlinx/serialization/descriptors/ContextAware.kt

@@ -86,7 +86,6 @@ internal fun SerialDescriptor.withContext(context: KClass<*>): SerialDescriptor
  * Descriptor that captures [kClass] and allows retrieving additional runtime information,
  * if proper [SerializersModule] is provided.
  */
-@OptIn(ExperimentalSerializationApi::class)
 private class ContextDescriptor(
     private val original: SerialDescriptor,
     @JvmField val kClass: KClass<*>

core/commonMain/src/kotlinx/serialization/descriptors/SerialDescriptors.kt

@@ -354,7 +354,6 @@ public inline fun <reified T> ClassSerialDescriptorBuilder.element(
     element(elementName, descriptor, annotations, isOptional)
 }
 
-@OptIn(ExperimentalSerializationApi::class)
 internal class SerialDescriptorImpl(
     override val serialName: String,
     override val kind: SerialKind,
@@ -363,6 +362,7 @@ internal class SerialDescriptorImpl(
     builder: ClassSerialDescriptorBuilder
 ) : SerialDescriptor, CachedNames {
 
+    @OptIn(ExperimentalSerializationApi::class)
     override val annotations: List<Annotation> = builder.annotations
     override val serialNames: Set<String> = builder.elementNames.toHashSet()
 

core/commonMain/src/kotlinx/serialization/internal/CollectionDescriptors.kt

@@ -1,14 +1,11 @@
 /*
  * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
-@file:OptIn(ExperimentalSerializationApi::class)
-
 package kotlinx.serialization.internal
 
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 
-@ExperimentalSerializationApi
 internal sealed class ListLikeDescriptor(val elementDescriptor: SerialDescriptor) : SerialDescriptor {
     override val kind: SerialKind get() = StructureKind.LIST
     override val elementsCount: Int = 1
@@ -107,7 +104,6 @@ internal const val HASH_MAP_NAME = "kotlin.collections.HashMap"
  *
  * Can be obtained from corresponding serializers (e.g. [ByteArraySerializer.descriptor])
  */
-@OptIn(ExperimentalSerializationApi::class)
 internal class PrimitiveArrayDescriptor internal constructor(
     primitive: SerialDescriptor
 ) : ListLikeDescriptor(primitive) {

core/commonMain/src/kotlinx/serialization/internal/NullableSerializer.kt

@@ -43,7 +43,6 @@ internal class NullableSerializer<T : Any>(private val serializer: KSerializer<T
     }
 }
 
-@OptIn(ExperimentalSerializationApi::class)
 internal class SerialDescriptorForNullable(
     internal val original: SerialDescriptor
 ) : SerialDescriptor by original, CachedNames {

core/commonMain/src/kotlinx/serialization/internal/PluginGeneratedSerialDescriptor.kt

@@ -13,7 +13,6 @@ import kotlinx.serialization.encoding.CompositeDecoder.Companion.UNKNOWN_NAME
  * Implementation that plugin uses to implement descriptors for auto-generated serializers.
  */
 @PublishedApi
-@OptIn(ExperimentalSerializationApi::class)
 internal open class PluginGeneratedSerialDescriptor(
     override val serialName: String,
     private val generatedSerializer: GeneratedSerializer<*>? = null,

formats/json/commonMain/src/kotlinx/serialization/json/JsonElementSerializers.kt

@@ -148,7 +148,6 @@ private object JsonLiteralSerializer : KSerializer<JsonLiteral> {
 internal object JsonObjectSerializer : KSerializer<JsonObject> {
 
     private object JsonObjectDescriptor : SerialDescriptor by MapSerializer(String.serializer(), JsonElementSerializer).descriptor {
-        @ExperimentalSerializationApi
         override val serialName: String = "kotlinx.serialization.json.JsonObject"
     }
 
@@ -173,7 +172,6 @@ internal object JsonObjectSerializer : KSerializer<JsonObject> {
 internal object JsonArraySerializer : KSerializer<JsonArray> {
 
     private object JsonArrayDescriptor : SerialDescriptor by ListSerializer(JsonElementSerializer).descriptor {
-        @ExperimentalSerializationApi
         override val serialName: String = "kotlinx.serialization.json.JsonArray"
     }
 
@@ -215,7 +213,6 @@ internal fun Encoder.asJsonEncoder() = this as? JsonEncoder
  * Returns serial descriptor that delegates all the calls to descriptor returned by [deferred] block.
  * Used to resolve cyclic dependencies between recursive serializable structures.
  */
-@OptIn(ExperimentalSerializationApi::class)
 private fun defer(deferred: () -> SerialDescriptor): SerialDescriptor = object : SerialDescriptor {
 
     private val original: SerialDescriptor by lazy(deferred)