[macOS] Eliminate unused OCMock includes (flutter#48031)

As part of the broader quest to reduce our dependence on OCMock in macOS
embedder tests, this removes #includes of OCMock into files where OCMock
is not actually used.

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Objective-C, Java style guides].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [X] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
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- [X] All existing and new tests are passing.

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https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
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shell/platform/darwin/macos/framework/Source/FlutterEmbedderKeyResponderTest.mm

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 #import <Foundation/Foundation.h>
-#import <OCMock/OCMock.h>
 
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterEmbedderKeyResponder.h"
 #include "flutter/shell/platform/embedder/test_utils/key_codes.g.h"

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

@@ -4,7 +4,6 @@
 
 #import <Cocoa/Cocoa.h>
 #import <Metal/Metal.h>
-#import <OCMock/OCMock.h>
 
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterSurface.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterSurfaceManager.h"

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

@@ -9,7 +9,6 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterTextInputSemanticsObject.h"
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController_Internal.h"
 
-#import <OCMock/OCMock.h>
 #import "flutter/testing/testing.h"
 
 namespace flutter::testing {

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

@@ -4,7 +4,6 @@
 
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.h"
 
-#import <OCMock/OCMock.h>
 #import "flutter/fml/synchronization/waitable_event.h"
 #import "flutter/testing/testing.h"