[ObjCRuntime] Remove unnecessary macOS code from .NET.

[rolfbjarne]

In Xamarin.Mac we have some code to support unusual ways of creating app
bundles while including Xamarin.Mac, and also dynamic loading of the Mono
runtime.

In .NET, we don't support these unusual app bundle creating logic for any
platform, and the Mono runtime isn't even an option for macOS, so exclude some
of the corresponding code from .NET.

This fixes a few warnings in NativeAOT about this code not being trimmer-safe.

[vs-mobiletools-engineering-service2]

💻 [PR Build] Tests on macOS M1 - Mac Ventura (13.0) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13.0) passed.

Pipeline on Agent
Hash: [PR build]

[vs-mobiletools-engineering-service2]

💻 [PR Build] Tests on macOS M1 - Mac Big Sur (11.5) passed 💻

✅ All tests on macOS M1 - Mac Big Sur (11.5) passed.

Pipeline on Agent
Hash: [PR build]

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

Legacy Xamarin (No breaking changes)

• iOS (no change detected)
• tvOS (no change detected)
• watchOS (no change detected)
• macOS (no change detected)

NET (empty diffs)

• iOS: (empty diff detected)
• tvOS: (empty diff detected)
• MacCatalyst: (empty diff detected)
• macOS: (empty diff detected)

✅ API diff vs stable

Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)
• Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

Legacy Xamarin (stable) vs .NET

• iOS: vsdrops gist
• tvOS: vsdrops gist
• macOS: vsdrops gist

✅ Generator diff

Generator diff is empty

Pipeline on Agent
Hash: f171702a2aa7873084c110d5f9167579cc98b399 [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build] Windows Integration Tests passed 💻

✅ All Windows Integration Tests passed.

Pipeline on Agent
Hash: f171702a2aa7873084c110d5f9167579cc98b399 [PR build]

[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: simulator tests.

🎉 All 235 tests passed 🎉

Tests counts

✅ bcl: All 69 tests passed. Html Report (VSDrops) Download
✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests: All 1 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 7 tests passed. Html Report (VSDrops) Download
✅ framework: All 8 tests passed. Html Report (VSDrops) Download
✅ generator: All 2 tests passed. Html Report (VSDrops) Download
✅ interdependent_binding_projects: All 7 tests passed. Html Report (VSDrops) Download
✅ install_source: All 1 tests passed. Html Report (VSDrops) Download
✅ introspection: All 8 tests passed. Html Report (VSDrops) Download
✅ linker: All 65 tests passed. Html Report (VSDrops) Download
✅ mac_binding_project: All 1 tests passed. Html Report (VSDrops) Download
✅ mmp: All 2 tests passed. Html Report (VSDrops) Download
✅ mononative: All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch: All 35 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ mtouch: All 1 tests passed. Html Report (VSDrops) Download
✅ xammac: All 3 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ xcframework: All 8 tests passed. Html Report (VSDrops) Download
✅ xtro: All 2 tests passed. Html Report (VSDrops) Download

Pipeline on Agent
Hash: f171702a2aa7873084c110d5f9167579cc98b399 [PR build]