Remove trigger and old column shared_runners_duration

Because we are unable to use change_column_type_concurrently, plus the tables ci_namespace_monthly_usages and ci_project_monthly_usages are not small, we need to apply guideline at https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#migrating-integer-primary-keys-to-bigint to convert column shared_runners_duration to bigint.

This is the third step to remove the trigger and the old column for shared_runners_duration