Clean up vsa_predefined_date_ranges feature flag

What does this MR do and why?

Removes all references to the vsa_predefined_date_ranges feature flag since it's been enabled by default, and refactors some of the related specs.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Please make sure that you're on the GitLab Ultimate plan.
2. Visit any group/project that has VSA data or follow these steps:
   1. run FILTER=customizable_cycle_analytics SEED_CUSTOMIZABLE_CYCLE_ANALYTICS=1 bundle exec rake db:seed_fu
   2. Visit the project/group value stream. The URL is present in the output of the command above.
3. Verify that the dropdown works correctly.
   • Predefined dates ranges return correct start and end dates, as well as number of days selected
   • Selecting a predefined date range hides date range picker
   • Selecting Custom option displays date range picker
4. Repeat Step 4 for CE as well (project level only)

Related to #438051 (closed)