[webauthn] Merged Pull Request: Trim unnecessary step from appId extension

kpaulh has just merged kpaulh's pull request 910 for https://github.com/w3c/webauthn:

== Trim unnecessary step from appId extension ==
It's not actually necessary to specify that the extension is only valid for getAssertion because it's already restricted by being part of AuthenticatorExtensionClientInputs.


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See https://github.com/w3c/webauthn/pull/910