ubd: Remove unused mutex 'ubd_mutex'

Commit fb5d1d38 ("ubd: open the backing files in ubd_add")

removed the last use of ubd_mutex.
Remove it.

Build and kernel startup test only.
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20240505001508.255096-1-linux@treblig.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/drivers/ubd_kern.c

@@ -106,7 +106,6 @@ static inline void ubd_set_bit(__u64 bit, unsigned char *data)
 #define DRIVER_NAME "uml-blkdev"
 
 static DEFINE_MUTEX(ubd_lock);
-static DEFINE_MUTEX(ubd_mutex); /* replaces BKL, might not be needed */
 
 static int ubd_ioctl(struct block_device *bdev, blk_mode_t mode,
 		     unsigned int cmd, unsigned long arg);