Drop Task.tags

A long time ago in an Arkindex release very far away, we could configure agents with tags to include and exclude. Those would be matched against tasks: agents that include some of the task's tags get priority, and agents that exclude any tag of the task will never get it assigned. This was meant to dedicate agents to ML or non-ML tasks, or even prioritize specific workers. We had a ml:[worker slug] tag assigned to tasks, and agents that want ML can include ml:* while those that don't accept that will exclude ml:*.

This feature was not used much and got dropped a while ago in ponos#82 on the agent side, but Task.tags still exists and is an empty array for every single task out there. We can drop that too, two years later…