Drop column `additional_amount_available` for CI monthly minutes

Problem

The table ci_namespace_monthly_usages contains a column additional_amount_available which was added when we created the table initially. This column was supposed to contain the migrated value of namespace.extra_shared_runners_minutes_limit to be tracked on a monthly basis.

Since the plans for additional minutes have changed to use AdditionalPacks model we don't need this column.

Solution

Drop the column entirely since it's not used at all and all the records contain the default 0 value.