device-dax: Drop an empty .remove callback

The dax core properly handles a dax driver not having a remove callback.
So drop it without changing the effective behaviour.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Link: https://lore.kernel.org/r/20210205222842.34896-5-uwe@kleine-koenig.org


Signed-off-by: Dan Williams <dan.j.williams@intel.com>