drm/amdkfd: Remove legacy code not acquiring VMs

ROCm user mode has acquired VMs from DRM file descriptors for as long
as it supported the upstream KFD. Legacy code to support older versions
of ROCm is not needed any more.

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