Revert "Fix test failures due to not null constraint on attachments ordering"

This reverts commit 9ff012b1495c1b50e284dba3c6fbd300e9e9dc6d.
Revert "Merge pull request #996 from alphagov/prevent-duplicate-attachment-orderings"

This reverts commit 3f567213fd77c7ae5ff643f608995bfdf8a3036f, reversing
changes made to a6e30e99269a66bb6affad3637bae37d79524c7b.