Use _deviceForNodeID:createIfNeeded: in more places. (#36602)

We had some manual lock-and-get code that can go away now.

src/darwin/Framework/CHIP/MTRDeviceController_Concrete.mm

@@ -1619,12 +1619,9 @@ - (void)invalidateCASESessionForNode:(NSNumber *)nodeID;
 
 - (void)operationalInstanceAdded:(NSNumber *)nodeID
 {
-    // Don't use deviceForNodeID here, because we don't want to create the
-    // device if it does not already exist.
-    os_unfair_lock_lock(self.deviceMapLock);
-    MTRDevice * device = [self.nodeIDToDeviceMap objectForKey:nodeID];
-    os_unfair_lock_unlock(self.deviceMapLock);
-
+    // If we don't have an existing MTRDevice for this node ID, that's fine;
+    // nothing to do.
+    MTRDevice * device = [self _deviceForNodeID:nodeID createIfNeeded:NO];
     if (device == nil) {
         return;
     }