[#1449] Fix extras deletion

The custom-fields module was actually disabling the fields when clicking
the delete extra cross button. This prevented the key and value from
being sent, which caused a validation error.
We now change only the class to make it look disabled, which should be
good enough.
[#1449] Remove unneeded tests
Merge branch 'master' into 1449-cant-delete-extra
Merge pull request #1449 from okfn/1449-cant-delete-extra

Can't delete extra