We aren't likely to try to make these changes any time soon, so cleaning out
these unnecessary methods which just return `this` will simplify things.
I was unable to find any calls to the `.eventTarget` getter in JS, which makes
sense, as the nsIThread type is only really used in JS as a wrapper around the
main thread in older code. Because of that, it has been removed as well.
Differential Revision: https://phabricator.services.mozilla.com/D166605