releasetools: Remove the unconditional fallback to bsdiff.

This CL uses the 'incomplete' tag to skip applying imgdiff to files with
incomplete block list. It's not the ideal fix to address the holes in
ext4 images, but would unhide other imgdiff issues covered by the
unconditional fallback.

Bug: 68016761
Test: Generate an incremental OTA package from images with incomplete
      block list. Check the imgdiff stats report.
Test: `python -m unittest test_blockimgdiff`
Change-Id: Ice77686414e70f5e42de35c1757fb31cf02e4fd4