Example cleanup: delete unsupported build targets & a broken menu item.

[timbotimbo2]

Description

Some cleanup of the example project. Basically the second half of #893

• Remove broken 'Native activity' menu option
• Remove folders for the macos and linux build targets, which are incompatible with Unity-as-a-Library.
• Hide the misleading windows export button in Unity, as the plugin doesn't currently support windows.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore