drbd: Remove unused conn_lowest_minor

conn_lowest_minor() last use was removed by 2011 commit
69a22773 ("drbd: Pass a peer device to a number of fuctions")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20241010204426.277535-1-linux@treblig.org


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>