Don't check the stderr output from bsdiff/imgdiff

bsdiff/imgdiff returns non-zero values correctly in case of failures.
So we don't need to check the stderr anymore. This avoids some false
reports of the diff program.

Bug: 72335938
Bug: 71505046
Test: Check the exit value of bsdiff/imgdiff in code search,
      generate a package for angler.

Change-Id: I18f0c3882a40a5288d6aee715713a05270e0db2b