MAINTAINERS: Orphanize the 'GDB stub' subsystem

Nobody is looking at those files, downgrade this subsystem as orphan.

Remove the qemu-devel@nongnu.org entry because the list is always
selected by the 'All patches CC here' section.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

MAINTAINERS

@@ -1815,8 +1815,7 @@ F: util/error.c
 F: util/qemu-error.c
 
 GDB stub
-L: qemu-devel@nongnu.org
-S: Odd Fixes
+S: Orphan
 F: gdbstub*
 F: gdb-xml/