Remove `resources :user_callouts` from `config/routes.rb`

What does this MR do and why?

• We have moved user callouts to users/callouts path via !75941 (merged) and can now remove this stub to the old path that handles stale clients/mixed deployments
• remove resource as we are now past the %14.6 milestone and mixed deployments on this route aren't an issue where this route could get invoked.
• logs for old endpoint hit
  • I still see about 1 per day being logged here, and I assume that those belong to some out of date clients.

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 #348096 (closed)