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.
`change` is meant for things which are automatically reversible. Putting this change into an `up` would make it explicit that there is a missing `down` migration.

Generally, migrations aren't reversed unless it's an aborted deploy. When they are needed, though, they can be a lifesaver.

In this case, if we needed to abort the deploy, the old code - which tries to set this field - would be running against a database without the column. This would mean that any changes to sites would fail because there would be no `global_http_status` method which is called by the old code.

`we're removing this column forever, right` - this is why I wasn't sure it mattered :) In this case, it's a simple change - if we really needed to, we could edit the migration on the server and add a `down` step.