When the client is destroyed, it's important that we correctly tear down
the frame-rpc event listeners (bound to the document's `onmessage`
event).

These event listeners are responsible for relaying messages from the
frame-rpc mechanism to various parts of the code on either side of the
frame boundary, including the `AnnotationSync` component. If the
frame-rpc channels aren't correctly torn down, they will hold a
reference to the `Bridge` object, which in turn holds a reference to the
`AnnotationSync` object, and thus old copies of `AnnotationSync` which
should have been garbage-collected will continue to receive events from
the brand new Hypothesis client.

The most visible result of this problem was that repeated activation and
deactivation of the client would result in increasing numbers of
highlights being drawn on annotated text, as reported in
hypothesis/h#3096.

Fixes hypothesis/h#3096.