Remove unneeded mutator_lock_ requires from fixed_up_dex_file

The REQUIRES_SHARED(art::Locks::mutator_lock_) is not required for the
FixedUpDexFile functions. Remove them to allow callers to avoid a
possible suspend point.

Test: Build

Change-Id: Id8e47f45026168f04be5df795766f6cfb5a69a16