Remove alsoKnownAs due to insufficient implementation experience.

[msporny]

This PR removes the alsoKnownAs feature as there were zero implementers out of 41 that implemented this feature. This PR presumes the WG will approve the following proposal: https://lists.w3.org/Archives/Public/public-did-wg/2021Jul/0005.html

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[talltree]

Two test reports for implementations of alsoKnownAs have now been submitted:

• https://github.com/w3c/did-test-suite/pull/186
• https://github.com/w3c/did-test-suite/pull/187

[msporny]

Two test reports for implementations of alsoKnownAs have now been submitted

Those tests reports have now been merged into the did-test suite, have been verified to come from two independent implementers, and the alsoKnownAs feature has met the minimum bar to remain in the DID Core specification.

Closing this PR (that removes alsoKnownAs) without merging based on the above rationale.