This removes the UNIFY_DIST and UNIFIED_BUILD variables, as well as the
--unify flag from the packager and UnifiedBuildFinder from mozpack. As a
result the STAGEPATH variable is never defined anymore, so its uses can
be removed as well.

test_unify.py is currently the only mozbuild/mozpack test that fails
without running configure first, and there isn't much point in fixing
tests for things that we don't actually use anymore.

MozReview-Commit-ID: F5q1FPW3Did

--HG--
extra : rebase_source : cadbd237f51c23ea1983135294521d628d16f0df