Remove HIP_STAGING_BUFFER

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/hip commit: ccc1bbe6b1]

projects/hip/include/hcc_detail/hip_hcc.h

@@ -55,7 +55,6 @@ extern int HIP_ATP_MARKER;
 extern int HIP_ATP;
 extern int HIP_DB;
 extern int HIP_STAGING_SIZE;   /* size of staging buffers, in KB */
-extern int HIP_STAGING_BUFFERS;    // TODO - remove, two buffers should be enough.
 extern int HIP_PININPLACE;
 extern int HIP_STREAM_SIGNALS;  /* number of signals to allocate at stream creation */
 extern int HIP_VISIBLE_DEVICES; /* Contains a comma-separated sequence of GPU identifiers */