[iOS] remove clear color hack. (#54451)

Fixes flutter/flutter#125640

This doesn't actually do anything, instead it forces opaque to false. If developers want opaque to be false, they can just set it directly.

Author: Jonah Williams

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

@@ -76,12 +76,6 @@ - (instancetype)initWithDelegate:(id<FlutterViewEngineDelegate>)delegate
     _delegate = delegate;
     _isWideGamutEnabled = isWideGamutEnabled;
     self.layer.opaque = opaque;
-
-    // This line is necessary. CoreAnimation(or UIKit) may take this to do
-    // something to compute the final frame presented on screen, if we don't set this,
-    // it will make it take long time for us to take next CAMetalDrawable and will
-    // cause constant junk during rendering.
-    self.backgroundColor = UIColor.clearColor;
   }
 
   return self;

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

@@ -51,10 +51,10 @@ - (void)testFlutterViewEnableSemanticsWhenIsAccessibilityElementIsCalled {
   XCTAssertTrue(delegate.callbackCalled);
 }
 
-- (void)testFlutterViewBackgroundColorIsNotNil {
+- (void)testFlutterViewBackgroundColorIsNil {
   FakeDelegate* delegate = [[FakeDelegate alloc] init];
   FlutterView* view = [[FlutterView alloc] initWithDelegate:delegate opaque:NO enableWideGamut:NO];
-  XCTAssertNotNil(view.backgroundColor);
+  XCTAssertNil(view.backgroundColor);
 }
 
 - (void)testIgnoreWideColorWithoutImpeller {