Remove unnecessary setting of empty arrays

We don't actually need to massage the params in this way anymore
because Rails 3.2 does some magic for us: it inserts a hidden input
for multi-selects, which will always get submitted, meaning we get an
empty array when all the options are deselected. See https://github.com/rails/rails/commit/faba406fa15251cdc9588364d23c687a14ed6885
Merge pull request #904 from alphagov/remove-redundant-code

Remove unnecessary setting of empty arrays