refactor: Remove unused generated templates

[worldbeater]

What kind of change does this PR introduce?

This PR removes the generated generic property validation classes that were reserved for future use. Since ReactiveUI.Validation now supports IObservable<T>-based validations (see #119), we are no longer planning to add extensions for those generic property validation classes, so removing the unused stuff might be reasonable.

What is the current behavior?

Currently, there are legacy generated classes available as undocumented public API of the library.

What is the new behavior?

Now, the generated classes are removed.

What might this PR break?

Those apps that are relying on the undocumented generated APIs might break at compile-time. The migration to the documented and more flexible ReactiveUI.Validation IObservable<T>-based APIs should be quite simple though, as described in README.md https://github.com/reactiveui/ReactiveUI.Validation#example

// A validator that validates multiple properties.
IObservable<bool> passwordsObservable =
    this.WhenAnyValue(
        x => x.Password,
        x => x.ConfirmPassword,
        (password, confirmation) => password == confirmation);

// Validator assignment to a property.
this.ValidationRule(
    vm => vm.ConfirmPassword,
    passwordsObservable,
    "Passwords must match.");

[glennawatson]

If it's like rxui and splat you'll need to add a nuget.config due to a ci server issue

[codecov]

Codecov Report

Merging #250 (87c772a) into main (e72eef1) will increase coverage by 23.17%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main     #250       +/-   ##
===========================================
+ Coverage   63.04%   86.21%   +23.17%     
===========================================
  Files          15       14        -1     
  Lines         893      653      -240     
===========================================
  Hits          563      563               
+ Misses        330       90      -240     

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[worldbeater]

If it's like rxui and splat you'll need to add a nuget.config due to a ci server issue

Yeah already done that in one of dependabot PRs #249