releasetools: Remove the sanity check on APEX payload key names.

Unlike container keys/certs, we'll always carry full names for APEX
payload signing keys.

Test: Run sign_target_files_apks.py on a target_files.zip with
      mismatching payload signing keys (e.g. shim apexes).
Test: `python -m unittest test_sign_target_files_apks`
Change-Id: Ifa329478f43b4f5ae665821564dbc14af7841330