MAINTAINERS: drop two usb-serial subdriver entries

Remove the remaining two obsolete usb-serial subdriver entries from
MAINTAINERS, which were missed in the recent purge by commit
f896b796 ("USB: Maintainers change for usb serial drivers").

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Matthias Urlichs <matthias@urlichs.de>