Delete generic_serial.h

Commit bb2a97e9 ("Staging: delete
generic_serial drivers") left generic_serial.h unused: nothing in the
tree includes it anymore. It is still exported, but since nothing in the
kernel uses the named constants this header provides, that seems
pointless. Delete this header too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>