Remove public Shutdown method on WriteHandler. (#8713)

We don't ever have long-lived WriteHandler instances, so don't need
external shutdown on them.

Fixes #8651

src/app/InteractionModelEngine.cpp

@@ -103,10 +103,7 @@ void InteractionModelEngine::Shutdown()
 
  for (auto & writeHandler : mWriteHandlers)
  {
- if (!writeHandler.IsFree())
- {
- writeHandler.Shutdown();
- }
+ VerifyOrDie(writeHandler.IsFree());
  }
 
  for (uint32_t index = 0; index < CHIP_IM_SERVER_MAX_NUM_PATH_GROUPS; index++)

src/app/WriteHandler.h

@@ -53,11 +53,6 @@ class WriteHandler
  */
  CHIP_ERROR Init(InteractionModelDelegate * apDelegate);
 
- /**
- * Shut down the ReadHandler. This terminates this instance
- * of the object and releases all held resources.
- */
- void Shutdown();
  /**
  * Process a write request. Parts of the processing may end up being asynchronous, but the WriteHandler
  * guarantees that it will call Shutdown on itself when processing is done (including if OnWriteRequest
@@ -99,6 +94,10 @@ class WriteHandler
  void MoveToState(const State aTargetState);
  void ClearState();
  const char * GetStateStr() const;
+ /**
+ * Clean up state when we are done sending the write response.
+ */
+ void Shutdown();
 
  Messaging::ExchangeContext * mpExchangeCtx = nullptr;
  WriteResponse::Builder mWriteResponseBuilder;