releasetools: Remove RemoveBackwardEdges().

RemoveBackwardEdges() was used only in BBOTA v1 (which has been
deprecated since O). v2+ calls ReverseBackwardEdges() and
ImproveVertexSequence().

Also remove the imgdiff tag of 'trimmed' that would be set through this
function() only.

Test: python -m unittest test_blockimgdiff
Test: Build an incremental non-A/B OTA.
Test: Code search shows no active user.
Change-Id: I3b58ae048a1fbc283269e70fdfa29eb8d184ede7