Remove select2 npm package

What does this MR do and why?

We have migrated all select2 usages to newer alternatives. We are thus ready to uninstall the dependency and all related code.

Screenshots or screen recordings

No visual change.

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 #374520 (closed)