Remove unused method com.google.protobuf.Protobuf.makeImmutable

This appears unused, and was package-private, so unlikely to have usages
outside of code we can see.

PiperOrigin-RevId: 816039862

Author: mhansen

java/core/src/main/java/com/google/protobuf/Protobuf.java

@@ -49,11 +49,6 @@ <T> void mergeFrom(T message, Reader reader, ExtensionRegistryLite extensionRegi
     schemaFor(message).mergeFrom(message, reader, extensionRegistry);
   }
 
-  /** Marks repeated/map/extension/unknown fields as immutable. */
-  <T> void makeImmutable(T message) {
-    schemaFor(message).makeImmutable(message);
-  }
-
   /** Checks if all required fields are set. */
   <T> boolean isInitialized(T message) {
     return schemaFor(message).isInitialized(message);