Reevaluate the need for the index on `issues.state`

This column only has a few possible values, meaning the index may not be all that useful; especially since we rarely filter issues by only this column. Personally I would prefer it if we could turn this column into a Rails enum (= an integer), that way the data we have to index becomes much smaller. Whether this is possible depends on whether we use state_machine (https://github.com/state-machines/state_machines-activerecord) for this state, and if so if we can perhaps drop that (or somehow make it work with Rails enums).