Remove MakeBundleNativeCodeExternal task and mkbundle functionality

[Copilot]

The mkbundle feature was only available in Xamarin.Android but was never implemented in .NET 6+. Since this repository now targets .NET 10, all mkbundle-related functionality should be removed.

Changes Made

• Removed main task: Deleted src/Xamarin.Android.Build.Tasks/Tasks/MakeBundleNativeCodeExternal.cs (274 lines)
• Removed tests: Deleted src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/MakeBundleNativeCodeExternalTests.cs (73 lines)
• Removed MSBuild integration: Removed UsingTask declaration from Xamarin.Android.Common.targets
• Removed error codes: Cleaned up XA5102, XA5103, and XA5201 from all localized resource files:
  • XA5102: "Conversion from assembly to native code failed. Exit code {0}"
  • XA5103: "NDK C compiler exited with an error. Exit code {0}"
  • XA5201: "NDK linker exited with an error. Exit code {0}"
• Removed documentation: Deleted MBN task prefix reference from Documentation/docs-mobile/messages/index.md
• Removed test project: Deleted entire tests/CodeGen-MkBundle/ directory containing mkbundle-specific test application

Verification

• Confirmed no remaining references to MakeBundleNativeCodeExternal in the codebase
• Preserved unrelated comments that mention mkbundle in general contexts (NDK tooling documentation)
• All changes are deletions with minimal modifications, maintaining code integrity

The removal is safe since mkbundle functionality was never available in .NET 6+ and this repository targets .NET 10.

Fixes #10207.

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[jonathanpeppers]

Will review this again after:

• Fix GitCommitInfo task to handle grafted repository state gracefully #10211

Which, will hopefully fix @copilot:

@copilot you can give a try if you like?

[jonathanpeppers]

@copilot try again now, with changes from main.

[Copilot]

@copilot try again now, with changes from main.

Fixed the blank line issues in the resource files and documentation. Removed extra blank lines from:

• Documentation/docs-mobile/messages/index.md
• src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs
• src/Xamarin.Android.Build.Tasks/Properties/Resources.resx

Changes committed in 61be1e7.