Remove redundant flag declarations in HwRouteScaleBenchmarkHelpers.h

Summary:
Remove flags and include from header instead:

```
DECLARE_bool(json);
DECLARE_bool(dsf_subscribe);
```

Reviewed By: srikrishnagopu

Differential Revision: D61942628

fbshipit-source-id: 824129957069c83bff2c5eced7e4d1a5ceef546b

fboss/agent/hw/benchmarks/BUCK

@@ -34,9 +34,11 @@ cpp_library(
         "HwRouteScaleBenchmarkHelpers.h",
     ],
     exported_deps = [
+        "//fboss/agent:agent_features",
         "//fboss/agent:core",
         "//fboss/agent:hw_switch",
         "//fboss/agent:utils",
+        "//fboss/agent/benchmarks:mono_agent_benchmarks",
         "//fboss/agent/hw/test:config_factory",
         "//fboss/agent/hw/test:hw_switch_ensemble_factory",
         "//fboss/agent/test:agent_ensemble",

fboss/agent/hw/benchmarks/HwRouteScaleBenchmarkHelpers.h

@@ -8,7 +8,9 @@
  *
  */
 
+#include "fboss/agent/AgentFeatures.h"
 #include "fboss/agent/HwSwitch.h"
+#include "fboss/agent/benchmarks/AgentBenchmarks.h"
 #include "fboss/agent/hw/test/ConfigFactory.h"
 #include "fboss/agent/hw/test/HwSwitchEnsemble.h"
 #include "fboss/agent/hw/test/HwSwitchEnsembleFactory.h"
@@ -26,9 +28,6 @@
 
 #include "fboss/agent/test/AgentEnsemble.h"
 
-DECLARE_bool(json);
-DECLARE_bool(dsf_subscribe);
-
 namespace {
 const auto kEcmpWidth = 512;
 const auto kEcmpGroup = 8;