[macOS] Merge FlutterSurfaceManager classes

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Previously, FlutterSurfaceManager was a protocol with two concrete implementations: FlutterGLSurfaceManager and FlutterMetalSurfaceManager. Most of the implementation was in a shared superclass, FlutterIOSurfaceManager, which called into the OpenGL or Metal-specific subclass when backend-specific operations (such as allocating textures) was required. It did so via a delegate pattern, wherein the subclasses both implemented the FlutterIOSurfaceManagerDelegate protocol that exposed the backend-specific functionality.

Now that only the Metal implementation remains, the delegate code can be inlined into the calling functions, and the class hierarchy can be squashed into a single concrete implementation class, FlutterSurfaceManager, similar to how it was originally implemented in #21525 before we had two backends.

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

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