Remove additional_amount_available column

Closed Avielle Wolfe requested to merge 344793-aw-drop-additional-amount-available into master

What does this MR do and why?

The additional_amount_available column on ci_namespace_monthly_usages was used during the transition to the new minute tracking logic. Now that the transition is over it can be removed.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #344793