Clean up ci_variable_drawer feature flag

What does this MR do and why?

This removes the ci_variable_drawer, which replaces the CI Variable modal with a drawer. The code for the modal is removed in this MR.

Rollout issue: #418005 (closed)

Feature issue: Starts with #410414 (closed)

Implementation Table

Iteration	MR
Setup drawer with feature flag	!126197 (merged)
Add form validations	!129684 (merged)
Ensure that CI variable actions are working with the drawer	!132067 (merged)
Rollout	#418005 (closed)
Clean up feature flag	👈 You are here!

Screenshots or screen recordings

Screen_Recording_2023-10-17_at_15.38.26

How to set up and validate locally

1. Make sure the ci_variable_drawer is disabled.
2. Go to Settings > CI/CD > Variables.
3. The drawer should show up for the following actions, which should be successful:
   • Adding a variable
   • Editing a variable (this will also add a delete button in the drawer)

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.