[linker] Remove C#8 Nullable attributes (#4056)

Fixes: #3911
Context: https://github.com/dotnet/roslyn/blob/7bc44488c661fd6bbb6c53f39512a6fe0cc5ef84/docs/features/nullable-metadata.md

C#8 [nullable reference types][0] use custom attributes to inform the
compiler about nullability information in the assembly.

These custom attributes shouldn't be needed when running on device,
as the C# language feature doesn't require modifications to the
runtime. Thus it should be safe to remove them.

[0]: https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/RemoveAttributes.cs

@@ -48,6 +48,9 @@ protected override bool IsRemovedAttribute (CustomAttribute attribute)
 			case "DebuggerTypeProxyAttribute":
 			case "DebuggerVisualizerAttribute":
 				return !DebugBuild && attr_type.Namespace == "System.Diagnostics";
+			case "NullableContextAttribute":
+			case "NullableAttribute":
+				return attr_type.Namespace == "System.Runtime.CompilerServices";
 			default:
 				return false;
 			}