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.

Link: https://lkml.kernel.org/r/20240319181842.249547-1-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Matthew Sakai <msakai@redhat.com>
Cc: Michael Sclafani <dm-devel@lists.linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: visitorckw

MAINTAINERS

@@ -6173,7 +6173,6 @@ F:	include/uapi/linux/dm-*.h
 
 DEVICE-MAPPER VDO TARGET
 M:	Matthew Sakai <msakai@redhat.com>
-M:	dm-devel@lists.linux.dev
 L:	dm-devel@lists.linux.dev
 S:	Maintained
 F:	Documentation/admin-guide/device-mapper/vdo*.rst