Don't delete e-mail validation sessions when the checking if the session was validated

This issue has been migrated from [#8086](https://github.com/matrix-org/synapse/issues/8086).

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With the current e-mail validation flow the user can be presented with a blank white page that says "Unknown session_id" pretty easily. The flow is something like the following:

1. The user is sent a validation e-mail.
2. The e-mail validation link is clicked and Synapse ensures that the data matches and [marks the e-mail as valid](https://github.com/matrix-org/synapse/blob/66f24449dd614b23ea4c572d8d613efeb129e4a2/synapse/rest/client/v2_alpha/account.py#L609-L612).
3. The user gets redirected to wherever their client wants based on the `next_link` parameter.
4. If a user has left Element running somewhere (maybe in a background tab, or on another device) it continually polls in the background and notices that the [e-mail is now valid](https://github.com/matrix-org/synapse/blob/83434df3812650f53c60e91fb23c2079db0fb5b8/synapse/handlers/ui_auth/checkers.py#L166-L183)). It then deletes the e-mail validation session.
5. The application continues with whatever is left in the registration flow (or logs the user in).

Unfortunately it seems that [some e-mail providers](https://support.google.com/mail/thread/16878288?hl=en) do content scanning on links, so they automatically visit a link *before the user sees it*. With that, the flow is modified a bit:

1. Step 1 from above happens as expected.
2. Before the user sees the e-mail the link is visited.
3. Since there's an app polling in the background, steps 2 - 4 happen.
4. The user urgently refreshes their inbox waiting for their validation email.
5. Finally the user is able to click on the validation link.
6. "Unknown session_id" error page 😢

I think the fix for this is to not delete the validation session during the UI Auth workflow and to delete them periodically (as we do with the UI auth sessions).