amdkfd: delete some dead code

This is dead code.  We don't need to unbind here, we can just return
directly.

Reviewed-by: Oded Gabbay <oded.gabbay@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>