docs: Remove stale ISDN parameters

docs: Remove stale ISDN parameters

The icn= and pcbit= parameters referenced drivers removed in
commit 02bbd9802da7 ("staging: i4l: delete the whole thing").

Remove the stale parameter entries and the now-unused ISDN tag
from the legend.

Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260502120206.1289126-1-costa.shul@redhat.com>