amd/amdkfd: drop unused KFD_IOCTL_SVM_FLAG_UNCACHED flag

Was leftover from GC 9.4.3 bring up and is currently
unused.  Drop it for now.

Cc: Philip.Yang@amd.com
Cc: rajneesh.bhardwaj@amd.com
Cc: Felix.Kuehling@amd.com
Reviewed-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>