[SE-0458] The unsafe expression and for..in effect is no longer experimental

Keep the experimental feature around for just a moment so we don't have
to pair commits across the swift-syntax and swift repositories.

Author: DougGregor

CodeGeneration/Sources/SyntaxSupport/ExprNodes.swift

@@ -183,7 +183,6 @@ public let EXPR_NODES: [Node] = [
   Node(
     kind: .unsafeExpr,
     base: .expr,
-    experimentalFeature: .unsafeExpression,
     nameForDiagnostics: "'unsafe' expression",
     children: [
       Child(

CodeGeneration/Sources/SyntaxSupport/StmtNodes.swift

@@ -287,7 +287,6 @@ public let STMT_NODES: [Node] = [
       Child(
         name: "unsafeKeyword",
         kind: .token(choices: [.keyword(.unsafe)]),
-        experimentalFeature: .unsafeExpression,
         isOptional: true
       ),
       Child(