Use Metal on iOS by default. (flutter#17431)

Fixes flutter#18208. All Metal for iOS related work items described in https://github.com/orgs/flutter/projects/5 have been completed.

shell/platform/darwin/ios/rendering_api_selection.mm

@@ -15,40 +15,17 @@
 
 namespace flutter {
 
-bool ShouldUseSoftwareRenderer() {
-  return [[[NSProcessInfo processInfo] arguments] containsObject:@"--force-software"];
-}
-
 #if FLUTTER_SHELL_ENABLE_METAL
 bool ShouldUseMetalRenderer() {
-  // If there is a command line argument that says Metal should not be used, that takes precedence
-  // over everything else. This allows disabling Metal on a per run basis to check for regressions
-  // on an application that has otherwise opted into Metal on an iOS version that supports it.
-  if ([[[NSProcessInfo processInfo] arguments] containsObject:@"--disable-metal"]) {
-    return false;
-  }
-
-  // If the application wants to use metal on a per run basis with disregard for version checks or
-  // plist based opt ins, respect that opinion. This allows selectively testing features on older
-  // version of iOS than those explicitly stated as being supported.
-  if ([[[NSProcessInfo processInfo] arguments] containsObject:@"--force-metal"]) {
-    return true;
-  }
-
-  // Flutter supports Metal on all devices with Apple A7 SoC or above that have been update to or
+  // Flutter supports Metal on all devices with Apple A7 SoC or above that have been updated to or
   // past iOS 10.0. The processor was selected as it is the first version at which Metal was
   // supported. The iOS version floor was selected due to the availability of features used by Skia.
   bool ios_version_supports_metal = false;
   if (@available(iOS 10.0, *)) {
     auto device = MTLCreateSystemDefaultDevice();
     ios_version_supports_metal = [device supportsFeatureSet:MTLFeatureSet_iOS_GPUFamily1_v3];
   }
-
-  // The application must opt-in by default to use Metal without command line flags.
-  bool application_opts_into_metal =
-      [[[NSBundle mainBundle] objectForInfoDictionaryKey:@"io.flutter.metal_preview"] boolValue];
-
-  return ios_version_supports_metal && application_opts_into_metal;
+  return ios_version_supports_metal;
 }
 #endif  // FLUTTER_SHELL_ENABLE_METAL
 
@@ -58,10 +35,6 @@ IOSRenderingAPI GetRenderingAPIForProcess() {
 #endif  // TARGET_IPHONE_SIMULATOR
 
 #if FLUTTER_SHELL_ENABLE_METAL
-  static bool should_use_software = ShouldUseSoftwareRenderer();
-  if (should_use_software) {
-    return IOSRenderingAPI::kSoftware;
-  }
   static bool should_use_metal = ShouldUseMetalRenderer();
   if (should_use_metal) {
     return IOSRenderingAPI::kMetal;