[ServiceWorker] Remove registration from Client? (#608)

I added this in the original design, but it doesn't fit with the static nature of the rest of the object.

I propose removing it, we can add it back later if needed, or something like `client.getRegistration()`.

@mounirlamouri does this work for you?

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/608