[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: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>