IB/hfi1: Remove unneeded empty check in hfi1_mmu_rb_unregister()

Checking if the rb tree is empty is redundant with the while loop which is
emptying the rb tree.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>