[paymentrequest] Don't have a <dfn> for "user agent" (#60)

It doesn't seem like this is the correct spec to define the entire concept of user agent, so having `<dfn id="dfn-user-agent">user agent</dfn>` in this spec is very strange.

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/paymentrequest/issues/60