Hide #ui-datepicker-div on on fields setup.

Hides the #ui-datepicker-div element on fields setup. Without it hidden, you get a thin white bar at the bottom of the page:

![screen shot 2014-08-15 at 09 37 40](https://cloud.githubusercontent.com/assets/4213522/3932322/6dd3cc3e-2468-11e4-9085-5ec6fde68220.png)

Changes tested in ACF 4.3.8 and 5.0.4.