ext4: remove duplicate remount check for JOURNAL_CHECKSUM change

rejection of, changing journal_checksum during remount.  One suffices.

While we're at it, remove old comment about the "check" option
which has been deprecated for some time now.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>