block: remove blkdev_write_begin() and blkdev_write_end()

block: remove blkdev_write_begin() and blkdev_write_end()

Remove blkdev_write_begin(), blkdev_write_end(), and their entries in
def_blk_aops. These have been unreachable since commit 487c607df790
("block: use iomap for writes to block devices") switched block device
buffered writes from generic_perform_write() to
iomap_file_buffered_write(), which bypasses aops->write_begin/end.

Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260525-blk-write-cleanup-v1-1-391c073e3831@columbia.edu
Signed-off-by: Jens Axboe <axboe@kernel.dk>