Remove legacy db entirely

[H-Shay]

This PR attempts to remove the (hopefully) outdated legacy DB from Sygnal, easing horizontal scaling.

[reivilibre]

Looks good!

[H-Shay]

I am unsure why the check_types run is failing, this check passes locally...

[reivilibre]

I think this is close now!

• please remove sygnal.log :)
• looks like you've been bitten by changes in Twisted (they added type annotations) which has left you with the trouble of cleaning up mypy after it. Fixing this was very noble of you, but (especially for code which you haven't touched in this PR), it's preferable to do it in a separate PR (this way, we're not waiting on this entire PR being approved before we fix CI [potentially affecting other PRs too]. It also makes reviewing this one a little bit less confusing :)). What we should have done is pin the versions of the dependencies so that this doesn't happen — if you fancy taking that on in another PR (probably using >=...,<... syntax so minor upgrades are still allowed? Refer to Synapse for an example), that would be appreciated and will save the next innocent person from having CI give them trouble :).
• my mistake for misleading you about the config_setup thing -- we want to reinstate it (since it's still overridden and used by subclasses) & the calls to it from the overriding methods (in case we ever add code to the function) -- just use the word pass in the parent's function, because that's the standard way of saying 'this is intentionally empty' -- you could just as well write 42 or super but it's unusual and confused me into thinking you didn't finish the line 😅 .

[H-Shay]

So I've made all the requested changes, and as such the CI is breaking on the types check, as expected. I opened a separate PR constraining the twisted version and fixing the type errors, perhaps we merge that and then update this branch before merging this branch? Not sure what the best way is!

[reivilibre]

just a handful of minor changes now, you'll soon see this one merged and released :)

[reivilibre]

perhaps we merge that and then update this branch before merging this branch? Not sure what the best way is!

Yup, that'd be fine. I think it's likely that this will be falling into place shortly

[callahad]

...while I'm poking at Sygnal, might as well update this, too ;)

[callahad]

(Note: I'm just doing rote things here, when merging please elide my Signed-off-by: and Co-authored-by: lines)

[reivilibre]

Great!