USB: serial: remove usb_serial_register and usb_serial_deregister

No one uses them anymore, they should be using the safer
usb_serial_register_drivers() and usb_serial_deregister_drivers()
functions instead.

Thanks to Alan Stern for writing these functions and porting all
in-kernel users to them.

Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>