releasetools: Remove the assertion of not allowing thumbprint in Treble world.

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