libbinder: Parcel.h: remove unused/internal funcs

acquire_object/release_object are not used outside of libbinder.

flatten_binder/unflatten_binder have changed in definition since they
were implemented in the header, so they are out of sync and would lead
to linking errors if they were used anyway.

This is part of an effort to remove linux/android/binder.h from
Parcel.h

Bug: 124524556
Test: (sanity) boot
Change-Id: I5633c53adf24d4217930afd7f4839dad9161e80f