Remove API Challenges modules

[eviltester]

Summary

This completes the API Challenges repo split from the Thingifier side after creating the standalone eviltester/apichallenges repository.

• removes challenger and challengerAuto from the Maven reactor
• deletes the old API Challenges source, tests, content, and Docker files from this repo
• updates Thingifier docs to point to https://github.com/eviltester/apichallenges
• keeps the reduced Thingifier reactor green and fixes two small static-analysis issues now exposed by the reduced build

Verification

• mvn -B clean test
• mvn -B spotless:check
• mvn -B -DskipTests verify
• mvn -B -pl standAloneTodoListManagerRestApiAuto -am test
• stale-reference search for challengerAuto, ChallengeMain, docker/apichallenges, and uk.co.compendiumdev.challenge

New repo

The extracted API Challenges repo is now at https://github.com/eviltester/apichallenges with Java CI and Docker CI passing on the latest runs.

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