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

[matrixbot]

This issue has been migrated from #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.
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). 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 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).