ISDN: eicon: Remove driver

I started looking at the history of this driver, and last time the
maintainer was active on the mailing list was when discussing how to
remove it. This was in 2012:

https://lore.kernel.org/lkml/4F4DE175.30002@melware.de/

It looks to me like this has in practice been an orphan for quite a while.
It's throwing warnings about stack size in a function that is in dire
need of refactoring, and it's probably a case of "it's time to call it".

Cc: Armin Schindler <mac@melware.de>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: David S. Miller <davem@davemloft.net>