Remove BindablePropertyAttribute.DefaultValue

[TheCodeTraveler]

Description of Change

This PR removes the DefaultValue property from [BindableProperty] for the following reasons now that it is not longer required thanks to #2987:

1. Setting the DefaultValue has been the largest source of pain and workarounds in BindablePropertySourceGenerator
   • You can see how the helper method creating the generated code continues to grow every time we discover a new not-yet-supported edge case
   • We can now remove that nasty helper method, saving maintenance costs and avoid future bugs from yet-to-be encountered edge cases
   • Initializers already support every edge case
2. The DefaultValue property only supported compile-time constants, while partial property initializer can use static readonly values
   • For example, [BindableProperty(DefaultValue = Colors.Transparent)] would generate a compiler error because Colors.Transparent is public static readonly
3. Improve the developer experience
   • It's a bit confusing to give developers two ways to initialize / set the default value of the bindable property
   • If we keep BindablePropertyAttribute.DefaultValue, we would need to write an additional Analyzer to generate a compiler error when we detect a developer is using both DefaultValue and the partial property initializer

PR Checklist

• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard

Additional information

This PR is dependent on #2987 and should not be merged before merging #2987

[ne0rrmatrix]

Do we want to add tests for most common system types that we will see used? Like DateTime and others? Or add them as we update our sample app?