Remove the currently_met field from a Need

This question on the form is redundant as it is confusing most of our users in testing, and the users which actually require this information already capture it through another process in the transition project.

Given we currently have no other users aside from Maslow using the Need API, I don't think it's worthwhile for us to maintain support for this field.

This should not be merged until after alphagov/maslow#35.