direct-io: Remove unused DIO_SKIP_DIO_COUNT logic

This flag was added by fe0f07d0 ("direct-io: only inc/deci
inode->i_dio_count for file systems") as means to optimise the atomic
modificaiton of the variable for blockdevices. However with the advent
of 542ff7bf ("block: new direct I/O implementation") it became
unused. So let's remove it.

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