This is no longer necessary as the Quantum DOM project is no longer
happening, and removing support simplifies various components inside of
IPDL.
As some code used the support to get a `nsISerialEventTarget` for an
actor's worker thread, that method was replaced with a method which
instead pulls the nsISerialEventTarget from the MessageChannel and
should work on all actors.
Differential Revision: https://phabricator.services.mozilla.com/D135411
