These two tests were previously added in
https://github.com/tomchristie/django-rest-framework/commit/7d79cf35b7be01b175d8c25276a4414e8144a16b
but we have now discovered that there are not actually two separate
cases, there was just a  bug in the code that made it look that way.

This also removes a redundant check to see if `DecimalValidator` was
defined.