[IPV6]: remove duplicate call to proc_net_remove

The file /proc/net/if_inet6 is removed twice.
First time in:
        inet6_exit
             ->addrconf_cleanup
And followed a few lines after by:
        inet6_exit
             -> if6_proc_exit
Signed-off-by: NDaniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -4288,8 +4288,4 @@ void __exit addrconf_cleanup(void)
 	del_timer(&addr_chk_timer);
 
 	rtnl_unlock();
-
-#ifdef CONFIG_PROC_FS
-	proc_net_remove(&init_net, "if_inet6");
-#endif
 }