netlink: cleanup tap related functions

Cleanups in netlink_tap code
 * remove unused function netlink_clear_multicast_users
 * make local function static

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>