USB: serial: drop unused core USB driver

Drop the usb-serial-core USB driver that was registered at module init
but then never used.

This was a remnant dating back to 2004 (!) when this struct usb_driver
was used for the generic driver; see commit bbc53b7d7322 ("USB: fix bug
where removing usb-serial modules or usb serial devices could oops") in
the tglx bitkeeper-history archive.

Note that every usb-serial driver (including the generic one) registers
its own USB (interface) driver along with its usb-serial bus drivers.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>