Remove P/invoke marshalling pregeneration from ReadyToRun.

[teo-tsirpanis]

https://github.com/dotnet/runtime/blob/4cc3808d9e3ad6a23fc8b0f1b167f532e9e2b067/docs/design/features/readytorun-pinvoke.md describes a mechanism where P/Invoke marshalling code for some common cases is generated ahead of time during ReadyToRun compilation.

Judging from the document this mechanism seems fragile and hard to maintain. Now that we have the LibraryImport source generator which is easier for ordinary R2R to take advantage of, this optimization is getting less relevant and maybe you should consider removing it for simplicity and ease of maintenance.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[trylek]

@jkoritzinsky, can you please clarify the delta between the PR you merged in and this issue i.o.w. what remains to be done here?

[jkoritzinsky]

The remaining work is to actually remove the P/Invoke emit infra from crossgen2 (including blittable stub support), but this work depends on #70109.

[mangod9]

@jkoritzinsky is there work still pending around this which will be done in 8 or should this be moved to later?