Code simplification/cleanup:
Remove stale fallback paths that uses something besides the unpinned engine.
Remove HIP_STAGING_BUFFER env var - now is const 2, 0 no longer has
special meaning.
Change-Id: I7d24cdd1067dd0c244e87b6a83897cb135d307e7
[ROCm/clr commit: 39750da9a5]