Remove Given the "foo" application has booted steps

These are a holdover from when we were running on passenger, and the
application workers were started on demand.  Now that we're using
Unicorn everywhere, the workers are running all the time.

This used to just send a HEAD request to the base URL for the app which
would cause passenger to spin up a worker if one wasn't already running.
Merge pull request #97 from alphagov/remove_has_booted_steps

Remove 'Given the "foo" application has booted' steps