[ObjC] Small cleanups to package level headers

- Reduce what headers the runtime import needs.
- Remove stale function prototype.
- Move one function to be full scoped to a single file.

PiperOrigin-RevId: 512073294
diff --git a/objectivec/GPBProtocolBuffers_RuntimeSupport.h b/objectivec/GPBProtocolBuffers_RuntimeSupport.h
index 804a68a..297173d 100644
--- a/objectivec/GPBProtocolBuffers_RuntimeSupport.h
+++ b/objectivec/GPBProtocolBuffers_RuntimeSupport.h
@@ -36,7 +36,6 @@
 // clang-format on
 
 #import "GPBDescriptor_PackagePrivate.h"
-#import "GPBExtensionInternals.h"
-#import "GPBMessage_PackagePrivate.h"
+#import "GPBMessage.h"
 #import "GPBRootObject_PackagePrivate.h"
 #import "GPBUtilities_PackagePrivate.h"