[iOS] Delete software rendering fallback in iOS embedder #190041

Currently, the iOS embedder has a fallback mechanism to use software rendering (IOSRenderingAPI::kSoftware / CALayer) if MTLCreateSystemDefaultDevice() fails to initialize a Metal device. Most of this logic lives in engine/src/flutter/shell/platform/darwin/ios/rendering_api_selection.mm.

This fallback was historically necessary for the iOS Simulator running in older CI environments or on host machines that lacked GPU paravirtualization (pre-macOS Catalina/Xcode 11).

We can safely remove this fallback and enforce IOSRenderingAPI::kMetal since:

• Impeller Migration: Flutter has fully migrated iOS to Impeller, which strictly requires a hardware-accelerated graphics API (Metal) and possesses no CPU software backend like Skia did. As the legacy Skia backend is removed from the engine, this Skia-dependent software fallback becomes dead code.
• Universal CI Metal Support: All modern CI environments capable of running Xcode 14+ (which requires macOS 12+) use Apple's modern virtualisation framework. This framework natively supports paravirtualised graphics and Metal on the iOS Simulator.
• Parity with macOS Embedder: The macOS desktop embedder (FlutterRenderer.mm) already hardcodes its configuration to Metal and successfully operates without any software rendering fallback.

This issue is the iOS equivalent of #156628, which tracks removing the software rendering fallback on Android.

Proposed Changes:

• Remove IOSRenderingAPI::kSoftware from rendering_api_selection.h and .mm.
• Remove the TARGET_OS_SIMULATOR fallback logic in GetRenderingAPIForProcess().
• Remove the CALayer return path in GetCoreAnimationLayerClassForRenderingAPI().
• Clean up any associated Skia software surface manager code specific to iOS.