Remove unused EquatableDiagnostic type (#403)

Since #401 factored diagnostics out of the incremental model,
EquatableDiagnostic is no longer used anywhere in the codebase.
Raw Diagnostic objects now flow through a separate pipeline that
preserves source locations for #pragma warning disable support.

Also removes the now-unused GetLocationTrimmed extension method
from RoslynHelpers, which was only called by EquatableDiagnostic.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: eiriktsarpalis

src/PolyType.Roslyn/Helpers/RoslynHelpers.cs

@@ -341,15 +341,6 @@ attributeData.ConstructorArguments[0].Value is INamedTypeSymbol typeParam &&
             return builderType is { IsGenericType: false } && builderMethod is not null;
         }
     }
-
-    /// <summary>
-    /// Get a location object that doesn't capture a reference to Compilation.
-    /// </summary>
-    public static Location GetLocationTrimmed(this Location location)
-    {
-        return Location.Create(location.SourceTree?.FilePath ?? string.Empty, location.SourceSpan, location.GetLineSpan().Span);
-    }
-
     public static ICollection<ITypeSymbol> GetSortedTypeHierarchy(this ITypeSymbol type)
     {
         if (type.TypeKind is not TypeKind.Interface)