Replace `Actor` `willDestroy` with `didDestroy`

As mentioned in #41710 (comment 2892234), we get the error message "JSWindowActorParent cannot send at the moment" whenever we close "about:torconnect" early and then continue to bootstrap in another tab.

This is because the callback to remove the observers is called willDestroy, but this was removed a while ago in bugzilla bug 1662771. We can use didDestroy instead.