releasetools: Clean up sign_target_files_apks.py.

Remove the obsolete workaround for API 24. Also make it pylint clean.

Test: 1. Sign a target_files.zip and get identical results.
      2. `pylint --rcfile=pylintrc sign_target_files_apks.py` gives 10.00/10.

Change-Id: I21736f959c5182486fd8ccebea9bbc594edef9fb
1 file changed