[webauthn] Merged Pull Request: remove unimplemented extensions (was: Update index.bs)

ve7jtb has just merged ve7jtb's pull request 1399 for https://github.com/w3c/webauthn:

== remove unimplemented extensions (was: Update index.bs) ==
This removes unimplemented extensions
Fixes #1386


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<a href="https://pr-preview.s3.amazonaws.com/ve7jtb/webauthn/pull/1399.html" title="Last updated on Apr 9, 2020, 1:37 AM UTC (83092b5)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/webauthn/1399/a636817...ve7jtb:83092b5.html" title="Last updated on Apr 9, 2020, 1:37 AM UTC (83092b5)">Diff</a>

See https://github.com/w3c/webauthn/pull/1399