[call] Fix field type on grpc_cq_completion (#29880)

* [call] Remove unused field from grpc_cq_completion

1. This field is unused, and therefore unnecessary
2. In https://github.com/grpc/grpc/pull/20119 this field was changed
   from a node (8 bytes) to a queue (80 bytes)

Removing this field should save 480 bytes per outstanding call on each
of the client and server.

* fix

src/core/lib/surface/completion_queue.h

@@ -41,7 +41,8 @@ extern grpc_core::DebugOnlyTraceFlag grpc_trace_pending_tags;
 extern grpc_core::DebugOnlyTraceFlag grpc_trace_cq_refcount;
 
 typedef struct grpc_cq_completion {
-  grpc_core::ManualConstructor<grpc_core::MultiProducerSingleConsumerQueue>
+  grpc_core::ManualConstructor<
+      grpc_core::MultiProducerSingleConsumerQueue::Node>
       node;
 
   /** user supplied tag */