Remove the logic hardcoding iteration dates in title

What does this MR do and why?

The changes in this MR are behind a feature flag (iteration_cadences).

When iterations are auto-generated (and updated) for a cadence, we've been inserting start and due dates of iterations into their titles. We will remove this behavior.

Example:

Currently (before this MR), an iteration starting on Oct 1, 2021 and ending on Oct 5, 2021 would be titled Iteration 1: Oct 1, 2021 - Oct 5, 2021.

Going forward the same iteration should be titled just Iteration 1.

(sidenote: we will be removing the numbering as well from title in the future)

This MR completes the backend side of #343523 (closed).

How to set up and validate locally

1. Enable the iteration cadences feature

   Feature.enable(:iteration_cadences)

2. Create a new iteration cadence for a group (ex. gitlab-org). Makes sure to check Automated scheduling (it should be checked by default).

3. Check that the created iterations for the cadence do not have dates in their titles.

• Iteration cadence list: http://gdk.test:3000/groups/gitlab-org/-/cadences

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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