MPEG4Extractor.cpp: Removed unused variable macro usage

Note: On host toolchains, __unused doesnt seem to be present. Hence
unused variables have been deprecated and code has been updated to
use br.skipBits() instead.

Test: Build libmp4extractor on Android
Bug: 151789258

Change-Id: I8c5ef6a626684e2982a8d9db8391e202c56af4f9