quota: remove superfluous DQUOT_OFF() in fs/namespace.c

We don't need to turn quotas off before remounting root ro, because
do_remount_sb() already handles this.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>