Remove BestFitMapping, CallingConvention, and ThrowOnUnmappableChar from GeneratedDllImportAttribute

[elinor-fung]

Also removed a few cases where #if DLLIMPORTGENERATOR_ENABLED is not needed.

Contributes to #61181

cc @AaronRobinsonMSFT @jkoritzinsky

[AaronRobinsonMSFT]

@elinor-fung Do we want to provide guidance in the compat doc about how to set calling conventions via the UnmanagedCallConvAttribute?

[elinor-fung]

Jeremy already did before:

runtime/docs/design/libraries/DllImportGenerator/Compatibility.md

Line 21 in f7be57f

[`CallingConvention`](https://docs.microsoft.com/dotnet/api/system.runtime.interopservices.dllimportattribute.callingconvention) will not be supported for `GeneratedDllImportAttribute`. Users will be required to use the new `UnmanagedCallConvAttribute` attribute instead. This attribute provides support for extensible calling conventions and provides parity with the `UnmanagedCallersOnlyAttribute` attribute and C# function pointer syntax. We will enable our conversion code-fix to automatically convert explicit and known calling convention usage to use the `UnmanagedCallConvAttribute`.

Or do you want something more?

[AaronRobinsonMSFT]

Or do you want something more?

@elinor-fung Nope. All good.