docs: isdn: remove hisax references from kernel-parameters.txt

The hisax driver got removed on 85993b8c ("isdn: remove hisax driver"),
but a left-over was kept at kernel-parameters.txt.

Fixes: 85993b8c ("isdn: remove hisax driver")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -1388,9 +1388,6 @@
 			Valid parameters: "on", "off"
 			Default: "on"
 
-	hisax=		[HW,ISDN]
-			See Documentation/isdn/README.HiSax.
-
 	hlt		[BUGS=ARM,SH]
 
 	hpet=		[X86-32,HPET] option to control HPET usage