releasetools: Remove the workarounds for mkyaffs2image.

Test: `m dist` on Linux and macOS.
Test: Use an existing target_files.zip for bullhead. Remove IMAGES/*
      and rebuild the images with 'add_img_to_target_files.py -a'. It
      generates idential images as in the original TF.zip.
Change-Id: I803656bf2c924dce53f2271dcb967a94fff48440