Remove `Clean worker caches` step in `_charm-quality-checks.yaml`

The `Clean worker caches` step in `_charm-quality-checks.yaml` was added to prevent the disk space of the runner from filling up for the GH runners. This was a temporary fix to unblock a few failing CIs in heavy charm itests.

This clean-up step should be removed or revisited (maybe it can also be a configurable step?) as soon as it is possible for the caller CIs to request a self-hosted runner.