Cleanup: Remove Exposed=ServiceWorker work-around from MessageChannel.idl

As of r173732 we no longer need it, as all interfaces exposed to
Worker should be now also available in ServiceWorker.

BUG=366063
TEST=http/tests/serviceworker/postmessage-msgport-to-client.html

Review URL: https://codereview.chromium.org/278283002

git-svn-id: svn://svn.chromium.org/blink/trunk@173862 bbb929c8-8fbe-4397-9dbb-9b2b20218538