Remove unused OperationTypeCodec. (#1746)

JAVA-5769

Author: vbabanin

driver-core/src/main/com/mongodb/client/model/changestream/ChangeStreamDocumentCodec.java

@@ -35,8 +35,6 @@
 @SuppressWarnings({"unchecked", "rawtypes"})
 final class ChangeStreamDocumentCodec<TResult> implements Codec<ChangeStreamDocument<TResult>> {
 
-    private static final OperationTypeCodec OPERATION_TYPE_CODEC = new OperationTypeCodec();
-
     private final Codec<ChangeStreamDocument<TResult>> codec;
 
     ChangeStreamDocumentCodec(final Class<TResult> fullDocumentClass, final CodecRegistry codecRegistry) {