`SecureContext` attribute is redundant for interfaces with `Exposed=ServiceWorker`

[saschanaz]

The latter should imply the former generally. And Firefox has a devtools feature to let service worker run on insecure http temporarily, so we can't have SecureContext.

(The interfaces in the Service Worker spec also just have the latter.)

[annevk]

I thought that we wanted these to be redundantly specified. Was this decided somewhere?