Delete checks for retired change waves

[ladipro]

Context

We still have checks for a change wave that's already out of rotation.

Changes Made

[MSBuild]::AreFeaturesEnabled('17.0') is now unconditionally true, let's remove the checks from Microsoft.Common.CurrentVersion.targets.

[rainersigwald]

I wonder if we should consider making a debug-only check in this property function to warn if there's an instance of a stale check like this.

[ladipro]

I wonder if we should consider making a debug-only check in this property function to warn if there's an instance of a stale check like this.

I had the same thought after creating the PR. Technically the assert is validating user input which is generally discouraged but since the feature is used pretty much only internally I think it makes sense to add it. Done.

[rainersigwald]

Technically the assert is validating user input which is generally discouraged but since the feature is used pretty much only internally I think it makes sense to add it.

I agree--and I think making it debug-only is a good mitigation in case someone else is using it.