IPoIB: Remove unnecessary test for NULL before debugfs_remove()

Fix checkpatch warning:

    WARNING: debugfs_remove(NULL) is safe this check is probably not required

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>