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

This issue has been migrated from [#6036](https://github.com/matrix-org/synapse/issues/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 https://github.com/matrix-org/matrix-spec/pull/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).

Start with synapse/handlers/room_member.py at the referenced lines 1037-1047 and 873-876, then compare the response with the Matrix identity specification and the linked review discussion. Determine whether the public_key and fallback_public_key handling is unused or incorrect; done means the legacy behavior is removed or corrected consistently with the specification.

Written by the indexing model from the issue text.