[macOS] Cleanup unnecessary references to Metal (#37733)

Removes a few unncessary references to Metal in the macOS embedder. Now
that Metal is the only supported rendering implementation for the macOS
embedder, specifying a class as a Metal variant is redundant.

Issue: flutter/flutter#108304
Issue: flutter/flutter#114445

shell/platform/darwin/macos/framework/Source/FlutterCompositorUnittests.mm

@@ -10,16 +10,16 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterViewProvider.h"
 #import "flutter/testing/testing.h"
 
-@interface FlutterViewMockProviderMetal : NSObject <FlutterViewProvider> {
+@interface FlutterViewMockProvider : NSObject <FlutterViewProvider> {
   FlutterView* _defaultView;
 }
 /**
- * Create a FlutterViewMockProviderMetal with the provided view as the default view.
+ * Create a FlutterViewMockProvider with the provided view as the default view.
  */
 - (nonnull instancetype)initWithDefaultView:(nonnull FlutterView*)view;
 @end
 
-@implementation FlutterViewMockProviderMetal
+@implementation FlutterViewMockProvider
 
 - (nonnull instancetype)initWithDefaultView:(nonnull FlutterView*)view {
   self = [super init];
@@ -57,7 +57,7 @@ - (nullable FlutterView*)getView:(uint64_t)viewId {
       })
       .andReturn(backingStoreMock);
 
-  return [[FlutterViewMockProviderMetal alloc] initWithDefaultView:viewMock];
+  return [[FlutterViewMockProvider alloc] initWithDefaultView:viewMock];
 }
 }  // namespace
 

shell/platform/darwin/macos/framework/Source/FlutterSurfaceManagerTest.mm

@@ -9,13 +9,13 @@
 #include "flutter/testing/testing.h"
 #include "gtest/gtest.h"
 
-@interface TestMetalView : NSView
+@interface TestView : NSView
 
 - (nonnull instancetype)init;
 
 @end
 
-@implementation TestMetalView
+@implementation TestView
 
 - (instancetype)init {
   self = [super initWithFrame:NSZeroRect];
@@ -32,7 +32,7 @@ - (instancetype)init {
 static FlutterSurfaceManager* CreateSurfaceManager() {
   id<MTLDevice> device = MTLCreateSystemDefaultDevice();
   id<MTLCommandQueue> commandQueue = [device newCommandQueue];
-  TestMetalView* metalView = [[TestMetalView alloc] init];
+  TestView* metalView = [[TestView alloc] init];
   CALayer* layer = reinterpret_cast<CALayer*>(metalView.layer);
   return [[FlutterSurfaceManager alloc] initWithDevice:device
                                           commandQueue:commandQueue