Add delete button to each target branch rule

What does this MR do and why?

Adds a delete button to the target branch rule in the settings page.

Screenshots or screen recordings

How to set up and validate locally

• Enable the target_branch_rules_flag flag
• Visit /gitlab-org/gitlab-test/-/settings/merge_requests
• Create a new rule
• Click Delete button on the rule

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.

#17909 (closed)