[w3c/ServiceWorker] Remove foreign fetch (#1188)

Discussed in https://github.com/w3c/ServiceWorker/issues/1173.

Due to problems with double-keying, unclear trial results, and unclear use-cases, we're going to remove foreign fetch from the spec (and fetch spec).

We can reexamine use-cases later and look to reintroduce it in another form once we have better data.

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