Remove C++ TRAP caching feature flag in favour of CLI v2.17.5 check.

[sashabu]

Merge / deployment checklist

• [ ] Confirm this change is backwards compatible with existing workflows.
• [ ] Confirm the readme has been updated if necessary.
• [ ] Confirm the changelog has been updated if necessary.

[henrymercer]

There are some CI errors unrelated to your PR. We have a couple of PRs up to fix that we're just waiting on reviews to merge.