Cleanup sampling test cases

[LotharSee]

Description

• Remove unused uniqueness_exceptions variable
• Simplify get_sampling_decision function signature
• Disable Ruby tests, given that /sample_rate_route/* isn't implemented in any of the Rails nor Sinatra app.
  Commits can be reviewed independently.

Motivation

I'm working on adding additional sampling tests cases to the suite. I'm starting with preliminary minor cleanups before opening PRs with the new logic.

Workflow

1. ⚠️⚠️ Create your PR as draft
2. Follow the style guidelines of this project (See how to easily lint the code)
3. Work on you PR until the CI passes (if something not related to your task is failing, you can ignore it)
4. Mark it as ready for review

Once your PR is reviewed, you can merge it! ❤️

Reviewer checklist

• Check what scenarios are modified. If needed, add the relevant label (run-parametric-scenario, run-profiling-scenario...). If this PR modifies any system-tests internal, then add the run-all-scenarios label (more info).
• CI is green
  • If not, failing jobs are not related to this change (and you are 100% sure about this statement)
• if any of build-some-image label is present
  1. is the image labl have been updated ?
  2. just before merging, locally build and push the image to hub.docker.com
• if a scenario is added (or removed), add (or remove) it in system-test-dasboard nightly