MAINTAINERS: Delete generic_serial section

commit bb2a97e9 ("Staging: delete generic_serial drivers")
removed the files, delete the section.

CC: Greg Kroah-Hartman <gregkh@suse.de>

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>