Remove the global_http_status column

- This is the third and final part of using `type` instead of `http_status`
  globally.
- I got a collation diff - I've `git add -p` only the parts of my
  `db/structure.sql` file relevant to adding this new migration.
This migration isn't automatically reversible - for example, it doesn't know what type the column should be. I'm in two minds about whether this matters in practice, but thought it worth pointing out.