Improve abidump generating logic for non-REL build

Optimize build by removing redundant edges from the dependency graph.
After this change abidump files (.sdump, .lsdump, .abidiff) are built
only if (1) building phony target `findlsdumps` or (2) reference dump
is present under prebuilts/, which is the case for REL branches.

This improves the build time by 1.5 minutes on my machine.

Also remove HEADER_ABI_DIFFS because it has no user anymore.

Bug: 162301547
Test: (on master) m && .sdump and .lsdump is not generated
Test: (on REL br) m && .sdump, .lsdump and .abidiff are generated
Change-Id: Ie040371d8fd57f91ec6f0cd4d37483167ef9c445