Unused legacy fallback code for public_key response to /_matrix/identity/v2/store-invite

[matrixbot]

This issue has been migrated from #6036.

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Noticed by @richvdh during review of https://github.com/matrix-org/synapse/pull/5979/files#r323956886

https://github.com/matrix-org/synapse/blob/c755955f335984dc6f97a269b57ad955f257ef8f/synapse/handlers/room_member.py#L1037-L1047

https://github.com/matrix-org/synapse/blob/c755955f335984dc6f97a269b57ad955f257ef8f/synapse/handlers/room_member.py#L873-L876

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Original comment:

while I'm here (it's not really relevant to the review, but): wtf is going on here?

1. public_key isn't specced anywhere, afaict
2. if public_key isn't set, we set fallback_public_key to public_keys[0] which, according to the spec, has a completely different shape to what we claim to return. (Edit: fixed by Corrections to the response format of /_matrix/identity/v2/store-invite matrix-org/matrix-spec#1486)

The calling code seems to imply that fallback_public_key is only used to populate some fields "For backwards compatibility", but said fields are in the spec.

I'm not suggesting changing anything here as part of this PR, but it looks like there's some bogosity, which suggests to me that the code is either unused (so can be killed) or broken (so should be fixed).