Since we stopped A/B testing the new Impact module in {T336203} on all Wikipedias, we should start the process of dropping the support of the `oldimpact` Homepage variant, which includes dropping A/B testing itself and dropping the old Impact module code as well.
(note we're A/B testing the new Impact module, but for technical reasons, the treatment group is actually called `oldimpact` and is given the old Impact module in the code; hence the confusing title)
==== Checklist
[X] Confirm with @KStoller-WMF that we will not need the old Impact module in the future
[X] Drop the `oldimpact` homepage variant and stop assigning it (makes the old Impact module class unused),
[ ] Drop the existing `oldimpact`rows in DB
[ ] Drop the old Impact related code
[ ] Possibly rename `NewImpact` to just `Impact`, since the distinction makes no sense to keep
[ ] Remove unnecessary feature flag `GEUseNewImpactModule`