Breaking Change: Stop generating the "newInstance" method. This is not used anymore.

PiperOrigin-RevId: 600123108

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

@@ -10,7 +10,6 @@ package com.google.protobuf;
 final class NewInstanceSchemaFull implements NewInstanceSchema {
   @Override
   public Object newInstance(Object defaultInstance) {
-    return ((GeneratedMessage) defaultInstance)
-        .newInstance(GeneratedMessage.UnusedPrivateParameter.INSTANCE);
+    return ((Message) defaultInstance).toBuilder().buildPartial();
   }
 }

src/google/protobuf/compiler/java/message.cc

@@ -357,15 +357,6 @@ void ImmutableMessageGenerator::Generate(io::Printer* printer) {
       "}\n"
       "\n");
 
-  printer->Print(variables,
-                 "@java.lang.Override\n"
-                 "@SuppressWarnings({\"unused\"})\n"
-                 "protected java.lang.Object newInstance(\n"
-                 "    UnusedPrivateParameter unused) {\n"
-                 "  return new $classname$();\n"
-                 "}\n"
-                 "\n");
-
   GenerateDescriptorMethods(printer);
 
   // Nested types