[generator] Remove generated, 32bits specific code paths for XM (#8444)

Product now only supports 64bits but the generator was still
producing some dual 32/64 code.

```
    Assembly    |    Profile    |   Before   |    After   |   Diff  |  %
----------------|---------------|------------|------------|---------|------
Xamarin.Mac.dll | full 64bits   | 22 348 800 | 22 082 560 | 266 240 | 1.2%
Xamarin.Mac.dll | mobile 64bits | 22 352 384 | 22 085 632 | 266 752 | 1.2%
```

Fix https://github.com/xamarin/xamarin-macios/issues/8441

src/generator.cs

@@ -3799,19 +3799,7 @@ public partial class Generator : IMemberGatherer {
 		int index64 = dual_enum ? 1 : 0;
 
 		if (CurrentPlatform == PlatformName.MacOSX) {
-			if (need_multi_path) {
-				print ("if (IntPtr.Size == 8) {");
-				indent++;
-				GenerateInvoke (x64_stret, supercall, mi, minfo, selector, args[index64], assign_to_temp, category_type, aligned && x64_stret, EnumMode.Bit64);
-				indent--;
-				print ("} else {");
-				indent++;
-				GenerateInvoke (x86_stret, supercall, mi, minfo, selector, args[0], assign_to_temp, category_type, aligned && x86_stret, EnumMode.Bit32);
-				indent--;
-				print ("}");
-			} else {
-				GenerateInvoke (x86_stret, supercall, mi, minfo, selector, args[0], assign_to_temp, category_type, aligned && x86_stret);
-			}
+			GenerateInvoke (x64_stret, supercall, mi, minfo, selector, args[index64], assign_to_temp, category_type, aligned && x64_stret, EnumMode.Bit64);
 			return;
 		}