Disable incomplete windows support.

[timbotimbo2]

Description

Windows support was noted as "coming soon" over 2 years ago.
As far as I know, it was never completed or functional.

This is likely because PlatformView support on Windows isn't complete in Flutter either.

This PR is some simple cleanup to avoid confusion:

• Remove windows directory from the example project. This avoids accidentally running debug on windows.
  This can be easily recreated using flutter create when needed again.
• Remove "coming soon" from the ReadMe.
• Comment the windows platform in pubspec.yaml to stop indicating to pub that windows is supported.

All progress in the plugin windows directory is kept, for any future work on windows support.

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