Remove very old draft ece-03 support

[jrconlin]

Data using this encoding format is very old and should no longer be supported. This may require some coordination with the Autopush Endpoint server to simply block incoming requests with appropriate messaging.

[jrconlin]

In case folk are curious, this is what the server sees:

mozilla-services/autopush-rs#266

[rfk]

If I'm understanding correctly, the ece-03 draft defines the aesgcm encoding, which the graph above suggests is (unfortunately!) the most commonly used. The unused aesgcm128 scheme seems to come from an earlier thomson-02 draft, and as noted in issue comment here I don't think it is supported by this crate.

I think that, happily, we don't have anything to do here (although Desktop does, ref Bug 1230038). But I could easily be lost in the web of specs here, and would appreciate a second opinion on the chain of reasoning here and in the issue comment linked above.