isdn: mISDN: remove a bogus reference to a non-existing doc
The mISDN driver was added on those commits:
0ca0b2a4655f ("Add mISDN DSP")
49d01f2fc7c8 ("Add mISDN core files")
656564fbea5f ("Define AF_ISDN and PF_ISDN")
0ad72545a21d ("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>