This commit:
- removes sendRpcMessage, which was unused;
- removes the CPOW argument to sendAsyncMessage, broadcastAsyncMessage, and
sendSyncMessage;
- removes the aIsSync argument used internally to distinguish sendRpcMessage
and sendSyncMessage;
- removes CPOW tests;
- updates the few remaining callsites that use more than 2 arguments in
sendAsyncMessage for the removal of the cpows argument.
Differential Revision: https://phabricator.services.mozilla.com/D71514
