isdn: remove isdn4linux

With all isdn4linux hardware drivers gone, this is only a wrapper around
CAPI to support old user space. However, from looking at the mailing
list, it seems that the last time anyone asked about it was in 2014,
when the upgrade from a linux-2.4 installation failed, and mISDN was
suggested as a replacement.

The largest public ISDN network (Deutsche Telekom) was supposed to be
shut down 2018, which must have drastically reduced the number of legacy
installations.

When we last discussed removing i4l in 2016, Karsten Keil suggested
revisiting this in 2018. I guess this is overdue.

Link: http://listserv.isdn4linux.de/pipermail/isdn4linux/2014-October/006165.html
Link: https://patchwork.kernel.org/patch/8484861/#17900371
Link: https://listserv.isdn4linux.de/pipermail/isdn4linux/2019-April/thread.html
Signed-off-by: Arnd Bergmann <[email protected]>