IB/hfi1: Consolidate __mmu_rb_remove and hfi1_mmu_rb_remove

__mmu_rb_remove was called in only 1 place which was a very simple
call site.  Combine this function into its caller.

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