MAINTAINERS: Remove incorrect M: tag for dm-devel@lists.linux.dev

The dm-devel@lists.linux.dev mailing list should only be listed under
the L: (List) tag in the MAINTAINERS file. However, it was incorrectly
listed under both L: and M: (Maintainers) tags, which is not accurate.
Remove the M: tag for dm-devel@lists.linux.dev in the MAINTAINERS file
to reflect the correct categorization.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>