Ever since protocol.js was added as a way to create DevTools actors, we've had
lots of confusion about the correct way to implement actor destruction. If your
actor's _parent_ was the legacy kind, you had to use `disconnect`. If it was
protocol.js, you had to use `destroy`.
There is no reason for this madness, which makes reasoning about destruction
quite hard. Here we rename `disconnect` to `destroy` so there is only one name
for every destruction path.
MozReview-Commit-ID: C1Yw9NfUUR2
