remove extra redundant channels setup in iOS embedding engine (#11886)

shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -93,7 +93,6 @@
   _pluginPublications = [NSMutableDictionary new];
   _platformViewsController.reset(new flutter::FlutterPlatformViewsController());
 
-  [self setupChannels];
   _binaryMessenger = [[FlutterBinaryMessengerRelay alloc] initWithParent:self];
 
   NSNotificationCenter* center = [NSNotificationCenter defaultCenter];

testing/scenario_app/ios/Scenarios/ScenariosTests/FlutterEngineTest.m

@@ -28,4 +28,23 @@
   XCTAssertNil(engine.isolateId);
 }
 
+- (void)testChannelSetup {
+  FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"test" project:nil];
+  XCTAssertNil(engine.navigationChannel);
+  XCTAssertNil(engine.platformChannel);
+  XCTAssertNil(engine.lifecycleChannel);
+
+  XCTAssertTrue([engine runWithEntrypoint:nil]);
+
+  XCTAssertNotNil(engine.navigationChannel);
+  XCTAssertNotNil(engine.platformChannel);
+  XCTAssertNotNil(engine.lifecycleChannel);
+
+  [engine destroyContext];
+
+  XCTAssertNil(engine.navigationChannel);
+  XCTAssertNil(engine.platformChannel);
+  XCTAssertNil(engine.lifecycleChannel);
+}
+
 @end