Remove documentation of non-existent sk_alloc arg

As you can see, there's no zero_it arg (in fact code always uses __GFP_ZERO).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>

net/core/sock.c

@@ -942,7 +942,6 @@ static void sk_prot_free(struct proto *prot, struct sock *sk)
  *	@family: protocol family
  *	@priority: for allocation (%GFP_KERNEL, %GFP_ATOMIC, etc)
  *	@prot: struct proto associated with this new sock instance
- *	@zero_it: if we should zero the newly allocated sock
  */
 struct sock *sk_alloc(struct net *net, int family, gfp_t priority,
 		      struct proto *prot)