remove option to disable vabc

Since dm-snapshot support is expired with android R + android S. We
should take out the option to disable vabc via ota_from_target_files.
For backwards compatibility, we still read from build rules to determine
if vabc is supported on both source and target target-files.

Bug: 416748567
Test: th
Change-Id: I39eb621ae1e8ae8f12c12f9306cf2fbf5ed6958f