Merge pull request grpc#24279 from donnadionne/fix

Remove unused variable.

Author: donnadionne

test/cpp/end2end/xds_end2end_test.cc

@@ -2287,7 +2287,6 @@ TEST_P(XdsResolverOnlyTest, CircuitBreaking) {
     Status status_;
   };
 
-  const char* kNewClusterName = "new_cluster";
   constexpr size_t kMaxConcurrentRequests = 10;
   SetNextResolution({});
   SetNextResolutionForLbChannelAllBalancers();