Staging: delete generic_serial drivers

No one has steped up to claim them, so as described in commit
4c377058 (tty: move obsolete and broken
generic_serial drivers to drivers/staging/generic_serial/), they are now
deleted from the system.

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@suse.de>