isdn: remove hisax driver

isdn: remove hisax driver

With the decline of ISDN, this seems to have become almost completely
obsolete, and even in the past years before that, almost all remaining
users appear to have used mISDN instead.

Birger Harzenetter noted that he is still using i4l/hisax to take
advantage of the 'divert' driver for call diversion, but otherwise uses
mISDN on the same hardware. This is a rare edge case as far as I
can tell, but we are still breaking an actively used work flow
(see https://xkcd.com/1172/).

We debated moving i4l/hisax to staging as an intermediate step, but as
he is not likely to change the setup, and that would just delay breaking
this use case.  The alternatives here are to stay on stable kernels
< 5.2, to create an external driver repository for isdn4linux, or to
add divert functionality to mISDN.

Cc: Birger Harzenetter <WIMPy@yeti.dk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>