remove `draft.isEnabled` setter from exotic draftMode wrappers (#77972)
For some reason `createExoticDraftMode[WithDevWarnings]` defines a
setter for `draft.isEnabled`. As far as i can tell, this setter is
unnecessary.
It doesn't seem like we ever allowed this:
```js
const draft = await draftMode()
draft.isEnabled = false
```
The `DraftMode` class had `isEnabled` as a pure getter from the start,
see #49096. You can also see that the previous implementation did the
same.
(also, the setter wasn't doing anything useful anyway, and arguably was
misleading -- it was defining the `isEnabled` property on the promise
itself, without actually performing `enable()/disable()`, so i can't see
how anyone could be relying on this)