releasetools: Remove the fallback-to-full logic.

The fallback-to-full logic doesn't look beneficial, but may hide away
real bugs, especially when it's on by default.

Test: Generate an incremental non-A/B OTA successfully.
Test: Inject bugs into script; observe that incremental OTA
      generation errors out instead of silently falling back.
Test: No internal users of the flag from code search.
Change-Id: Ifc627d898552c0bd72463e8c107e36610421608e