MINOR: backend: use conn_full_close() instead of conn_force_close()

There's no point in using conn_force_close() in outgoing connect()
since XPRT_TRACKED is not set so both functions are equivalent.

src/backend.c

@@ -1106,7 +1106,7 @@ int connect_server(struct stream *s)
 		/* we may have to release our connection if we couldn't swap it */
 		if (old_conn && !old_conn->owner) {
 			LIST_DEL(&old_conn->list);
-			conn_force_close(old_conn);
+			conn_full_close(old_conn);
 			conn_free(old_conn);
 		}
 	}