Remove FTUE onboarding as it is incompatible with SSO/OIDC

[t3chguy]

Fixes #26025

[dbkr]

I have no particular attachment to this screen, but the linked issue doesn't obviously come to the conclusion to kill it? We must be able to differentiate new logins from new registrations as I thought we use that to decide whether to set up cross signing or not?

[t3chguy]

I have no particular attachment to this screen, but the linked issue doesn't obviously come to the conclusion to kill it? We must be able to differentiate new logins from new registrations as I thought we use that to decide whether to set up cross signing or not?

The conclusion to the issue came in a call today between @pmaier1 & @daniellekirkwood - we may revisit similar onboarding next year, for now we did not wish to pursue the spec work & backend work necessary for the server to signal new vs existing account to the client.

[pmaier1]

We must be able to differentiate new logins from new registrations as I thought we use that to decide whether to set up cross signing or not?

Is that actually the case? That would certainly be a blocker. I would have thought we'd just use the cross-signing state to decide whether to set it up or not (like, if it's not set up yet, set it up)?

[dbkr]

Just checked and apparently I was wrong: we set up cross signing on any new registration or login. Would it be worth a quick note on that issue to say where this was decided, just so we all remember?