The claim in the original CL doesn't seem to hold. We may still have
devices using both of thumbprint and Treble compatibility check.
Bug: 111640956
Test; python -m unittest test_ota_from_target_files
Test: Build an incremental package for a target using thumbprints.
Change-Id: Ia491d5279901c2385d1c30802abcb99d65d919cc