isdn: mISDN: remove a bogus reference to a non-existing doc

The mISDN driver was added on those commits:

	960366cf ("Add mISDN DSP")
	1b2b03f8 ("Add mISDN core files")
	04578dd3 ("Define AF_ISDN and PF_ISDN")
	e4ac9bc1 ("Add mISDN driver")

None of them added a Documentation/isdn/mISDN.cert file.
Also, whatever were supposed to be written there on that time,
probably doesn't make any sense nowadays, as I doubt isdn would
have any massive changes.

So, let's just get rid of the broken reference, in order to
shut up a warning produced by ./scripts/documentation-file-ref-check.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>