[context] Move SubchannelCallTrackerInterface to arena based context (#36780)

Closes #36780

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36780 from ctiller:ctx6 b0867ad028
PiperOrigin-RevId: 639040058

src/core/lib/channel/context.h

@@ -59,9 +59,6 @@ typedef enum {
   /// the server.
   GRPC_CONTEXT_BACKEND_METRIC_PROVIDER,
 
-  /// A LoadBalancingPolicy::SubchannelCallTrackerInterface
-  GRPC_SUBCHANNEL_CALL_TRACKER_INTERFACE,
-
   /// Special Google context
   GRPC_CONTEXT_GOOGLE,
 

src/core/load_balancing/lb_policy.h

@@ -484,16 +484,11 @@ class LoadBalancingPolicy : public InternallyRefCounted<LoadBalancingPolicy> {
   ChannelArgs channel_args_;
 };
 
-namespace promise_detail {
-
 template <>
-struct OldStyleContext<LoadBalancingPolicy::SubchannelCallTrackerInterface> {
-  static constexpr grpc_context_index kIndex =
-      GRPC_SUBCHANNEL_CALL_TRACKER_INTERFACE;
+struct ArenaContextType<LoadBalancingPolicy::SubchannelCallTrackerInterface> {
+  static void Destroy(LoadBalancingPolicy::SubchannelCallTrackerInterface*) {}
 };
 
-}  // namespace promise_detail
-
 }  // namespace grpc_core
 
 #endif  // GRPC_SRC_CORE_LOAD_BALANCING_LB_POLICY_H