Remove unsupported fields from `GeneratedDllImport`

[elinor-fung]

• Remove BestFitMapping, CallingConvention, and ThrowOnUnmappableChar from GeneratedDllImportAttribute #61182
• Remove GeneratedDllImportAttribute.PreserveSig #65164

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.

[jkotas]

Do we have a plan on what to do about CharSet, CharSet.Ansi in particular?

CharSet.Ansi makes no sense on non-Windows. I see that the source generator requires you to explicitly specify CharSet when marshaling strings. At minimum, would it make sense to drop this requirement so that people on non-Windows do not have to write CharSet.Ansi and scratch their head wondering what it means?

[AaronRobinsonMSFT]

Do we have a plan on what to do about CharSet, CharSet.Ansi in particular?

@jkotas Yep. The Interop team was discussing that. One of our current ideas involves the Encoding class and completely ignores the CharSet for GeneratedDllImport scenarios. We are still in preliminary discussions about it but will propose something soon.

[jkotas]

One of our current ideas involves the Encoding class

I like this idea. It would allow us to handle all less common cases, without hardcoding them. But I think we should also have less verbose shortcuts or defaults for the most common cases (UTF16 on Windows, UTF8 on Unix, etc.). CharSet may be useful for these shortcuts.