Remove select2 logic from ee_mirror_repos.js

What does this MR do and why?

This cleans up a bit of code that became obsolete after we had migrated a select field to a normal text input in !98824 (merged).

Screenshots or screen recordings

No visual change.

How to set up and validate locally

The affected field is located in a project's Repository settings at /:namespace/:project/-/settings/repository.

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.

Related to #374016 (closed)