Kconfig: remove dangling references to the deleted file

Commit 202dc3fc (Documentation: remove
obsolete networking/multicast.txt file) deleted the obsolete file. After
the file has been removed, clean up a couple of places where references
to the deleted file were made so that users wouldn't be confused when
they consult the Help menu.

Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>