Remove unused ChallengeSweeper and ChallengeSignupSweeper

Description

As far as I can tell, these sweepers haven't been used for years. Both classes appear to be updating cache keys that aren't used anywhere else, and the only call to cache_sweeper I can find for either of them was removed in 2010. Let's remove them.

No manual QA required.

DeployedToBeta

Assignee

ticking instant

Reporter

ticking instant

Roadmap

Challenges

Priority

Low

Affects versions

Fix versions

Components

BackEnd

Difficulty

Medium

Milestone

Internal 0.9