ubifs: remove manual O_SYNC handling

generic_file_aio_write already calls into ->fsync to handle O_SYNC/O_DSYNC.
Remove the duplicate call to ubifs_sync_wbufs_by_inode which is already
covered by ubifs_fsync.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>