Remove GlobalTagStatus and GlobalTagType objects

[paullaycock]

"Types" were originally foreseen to support different types of behaviours of global tags. Now we have all of the needed behaviours supported without needing to switch on this it seems likely that it can be removed.
"Status" we only have "locked" and "unlocked" and it's not obvious we want to support more, so again it seems likely we can remove the table and retain status as a GT object attribute, and retain gt_change_status endpoint.

Not urgent and it would be good to see that we don't need finer granularity control of behaviour before removing either of these, but this would clean things up a bit.