Remove unused AjaxLoadingSpinner

What does this MR do and why?

Remove unused AjaxLoadingSpinner

The last remaining instance of .js-ajax-loading-spinner was removed in !77231 (merged), so this should be safe to delete entirely.

Screenshots or screen recordings

n/a

How to set up and validate locally

1. Verify .js-ajax-loading-spinner is no longer present in the codebase.

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 #352510 (closed)