Undo the ILLink/ILC enum-field substitution workaround from #26371 once dotnet/runtime#131601 is fixed

[rolfbjarne]

Background

PR #26371 works around an incompatibility between ILLink and ILC (NativeAOT) in how they parse the substitution value for an enum-typed field in ILLink.Substitutions.xml:

Tool	Accepted value
ILLink	the enum member's name (e.g. DEVICE / SIMULATOR)
ILC	the numeric value (e.g. 0 / 1)

There is no single value that works for both, and each reports IL2015 for the other's format. This surfaced on .NET 11 with NativeAOT, where ILLink is skipped entirely and ILC parses our substitution XML itself for the first time.

This is tracked upstream as dotnet/runtime#131601.

The workaround to undo

In PR #26371 we specified both variants of the ObjCRuntime.Runtime.Arch field substitution, in two separate <type> blocks selected by a new ObjCRuntime.IsILCompiler feature switch, and pass --feature:ObjCRuntime.IsILCompiler=true only to ILC.

Once dotnet/runtime#131601 is fixed (i.e. ILLink and ILC agree on the accepted substitution value format), we should undo the workaround:

• Remove the duplicated <type ... feature="ObjCRuntime.IsILCompiler" ...> blocks in src/ILLink.Substitutions.iOS.xml and src/ILLink.Substitutions.tvOS.xml (and any others that gained them), keeping a single Arch field substitution with whichever value both tools accept.
• Remove the --feature:ObjCRuntime.IsILCompiler=true IlcArg from _XamarinComputeIlcCompileInputs in dotnet/targets/Xamarin.Shared.Sdk.targets, along with the now-unused ObjCRuntime.IsILCompiler feature switch plumbing.

Validation

Ensure tests/dotnet/MySimpleApp/{iOS,tvOS} still publish cleanly both with and without -p:PublishAot=true, including the configuration where both ILLink and ILC process the XML.

References

• Workaround PR: [src] Use numeric values for the Runtime.Arch field substitution. #26371
• Upstream issue: ILLink and ILC disagree about how to specify enum values in ILLink.Substitutions.xml runtime#131601