staging: remove isdn capi drivers

As described in drivers/staging/isdn/TODO, the drivers are all
assumed to be unmaintained and unused now, with gigaset being the
last one to stop being maintained after Paul Bolle lost access
to an ISDN network.

The CAPI subsystem remains for now, as it is still required by
bluetooth/cmtp.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20191210210455.3475361-1-arnd@arndb.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>