tls: fix compiler warning of unused variable

src/modules/tls/tls_mod.c

@@ -588,8 +588,6 @@ static OSSL_LIB_CTX *new_ctx;
 #endif
 static int mod_child(int rank)
 {
-	int k;
-
 	if(tls_disable || (tls_domains_cfg == 0))
 		return 0;