Revert^2 "releasetools: check_target_files_signatures.py checks APEXes."

This reverts commit 5516d37f.

The previous issue in unzipping non-matching files has been addressed
with commit a49054ca2f2959f50f3188914ec0faebc90ebcbe. This CL rolls
forward to allow dumping container certifcates for APEXes.

Bug: 128848294
Test: Run check_target_files_signatures.py on target_files.zips w/ and
      w/o APEX files.
Change-Id: I662aab3d96fc40ac8e5e206e32b73ac763220b70