chore: Remove property for UAP splash entity

[lukeblevins]

GitHub Issue (If applicable): #

PR Type

What kind of change does this PR introduce?

What is the current behavior?

We currently allow users of ExtendedSplashScreen to get/set a SplashScreen object (Windows.ApplicationModel.Activation) which appears to not be implemented for Uno.UI targets.

What is the new behavior?

Remove ExtendedSplashScreen.SplashScreen property

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Tested the changes where applicable:
  • UWP
  • WinUI
  • iOS
  • Android
  • WASM
  • MacOS
• Updated the documentation as needed:
  • General Doc Update
  • Controls Doc Update
  • Extensions Doc Update
  • controls-styles.md
  • lightweight-styling.md (LightWeight Styling Resource Keys)
• Runtime Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Associated with an issue (GitHub or internal)
• Commits must be following the Conventional Commits specification.

Other information

Internal Issue (If applicable):