Remove unused ci_triggers.ref column

What does this MR do and why?

Related to Trigger Token's ref attribute is not used (#299985 - closed)

The column ci_triggers.ref was ignored in 15.2. We can safely remove the column now.

Database migrations

$ rails db:migrate:redo:ci

ci: == [advisory_lock_connection] object_id: 275380, pg_backend_pid: 11292
ci: == 20230510130050 RemoveCiTriggersRefColumn: reverting ========================
ci: -- add_column(:ci_triggers, :ref, :string, {:if_not_exists=>true})
ci:    -> 0.1962s
ci: == 20230510130050 RemoveCiTriggersRefColumn: reverted (0.3075s) ===============

ci: == [advisory_lock_connection] object_id: 275380, pg_backend_pid: 11292
ci: == [advisory_lock_connection] object_id: 278800, pg_backend_pid: 11568
ci: == 20230510130050 RemoveCiTriggersRefColumn: migrating ========================
ci: -- remove_column(:ci_triggers, :ref)
ci:    -> 0.0018s
ci: == 20230510130050 RemoveCiTriggersRefColumn: migrated (0.0288s) ===============

ci: == [advisory_lock_connection] object_id: 278800, pg_backend_pid: 11568

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.