Delete the FF ci_variable_for_group_gitlab_deploy_token

What does this MR do and why?

This MR removes the feature flag ci_variable_for_group_gitlab_deploy_token.

It's been enabled by default since v15.1.

• Bug issue: #214014 (closed)
• Introduced in: !88696 (merged)
• Enabled by default: !90341 (merged)
• Rollout issue: #363621 (closed)

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.