Revert "Allow overwriting virtual properties via setting `set` to `None`."

This reverts commit 0a8adfca6e5fce8835552c1c7aba063649ba3aeb.

core/src/avm1/object.rs

@@ -63,16 +63,9 @@ impl<'gc> Property<'gc> {
         new_value: Value<'gc>,
     ) {
         match self {
-            Property::Virtual {
+            Property::Virtual { set, .. } => {
-                set, attributes, ..
-            } => {
                 if let Some(function) = set {
                     function(avm, context, this, &[new_value]);
-                } else if !attributes.contains(ReadOnly) {
-                    *self = Property::Stored {
-                        value: new_value,
-                        attributes: *attributes,
-                    };
                 }
             }
             Property::Stored {