netfilter: conntrack: Remove useless assignment statements

The old_size assignment here will not be used anymore

The clang_analyzer complains as follows:

Value stored to 'old_size' is never read
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_core.c

@@ -2588,7 +2588,6 @@ int nf_conntrack_hash_resize(unsigned int hashsize)
 			hlist_nulls_add_head_rcu(&h->hnnode, &hash[bucket]);
 		}
 	}
-	old_size = nf_conntrack_htable_size;
 	old_hash = nf_conntrack_hash;
 
 	nf_conntrack_hash = hash;