The staging isdn drivers are gone, and CONFIG_BT_CMTP is now
the only user. This means a lot of the code in the subsystem
has no remaining callers and can be removed.

Change the capi user space front-end to be part of kernelcapi,
and the combined module to only be compiled if BT_CMTP is
also enabled, then remove the interfaces that have no remaining
callers.

As the notifier list and the capi_drivers list have no callers
outside of kcapi.c, the implementation gets much simpler.

Some definitions from the include/linux/*.h headers are only
needed internally and are moved to kcapi.h.
Acked-by: David Miller <davem@davemloft.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20191210210455.3475361-2-arnd@arndb.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>