f2fs: remove redundant comment of unused wio_mutex

Commit 089842de

 ("f2fs: remove codes of unused wio_mutex") removes codes
of unused wio_mutex, but missing the comment, so delete it.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>