releasetools: Remove the dead code for blockimgdiff_version <= 2.

The code is on infeasible path since we already have assertions in
common.BlockDifference().

Also remove the dead code that checks for OPTIONS.info_dict, as we
already set that in ota_from_target_files.main(), for both of A/B and
non-A/B.

Test: Generate incremental OTAs w/ and w/o the CL, and get identical
      packages.
Change-Id: Ifb8fc101e78f5ce58c60c8e49028b66ce0d20246