Remove host compatibility mode for `runtimes.json`

I was just reviewing the RID simplication design. I'm in the process of (finally) merging that spec given that we've been shipping that design for multiple years now. It occured to me that .NET 11 won't have support for most (all?) of the versions in `runtimes.json`. In the interests of reducing product complexity, I propose we remove this support. It doesn't serve any real purpose at this point. If we choose to remove this support, I'll open a matching issue for NuGet. The same logic applies.

- https://github.com/dotnet/designs/pull/260/.
- https://github.com/dotnet/runtime/blob/313982dff3457c8291e8b88cd99785f6920319f0/src/libraries/Microsoft.NETCore.Platforms/src/runtime.json

@elinor-fung