Adds controller action to delete an experiment

What does this MR do and why?

Users can't delete experiments, this is the first MR with the backend change to enable it. Gated behind :ml_experiment_tracking feature flag. No UX/UI changes.

Queries

DELETE FROM "ml_experiments" WHERE "ml_experiments"."id" = 25

https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/16950/commands/57606

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.