Remove deprecated contentTypeLocked field

Removed deprecated contentTypeLocked field to improve code clarity.

Author: hyperxpro

client/src/main/java/org/asynchttpclient/RequestBuilderBase.java

@@ -93,12 +93,6 @@ public abstract class RequestBuilderBase<T extends RequestBuilderBase<T>> {
     protected @Nullable Charset charset;
     protected ChannelPoolPartitioning channelPoolPartitioning = ChannelPoolPartitioning.PerHostChannelPoolPartitioning.INSTANCE;
     protected NameResolver<InetAddress> nameResolver = DEFAULT_NAME_RESOLVER;
-    // Flag to track if Content-Type was explicitly set by user (should not be modified)
-    // Keep this field protected for binary compatibility with subclasses compiled
-    // against earlier versions that may have accessed the field directly. New
-    // code should use the doContentTypeLock/resetContentTypeLock/isContentTypeLocked
-    // API instead.
-    @Deprecated
     protected boolean contentTypeLocked;
 
     /**