Do not change the z-index of the modal window

The resize handle issue only occurs in Chrome and appears to be a bug: https://code.google.com/p/chromium/issues/detail?id=158426

Since setting the z-index can cause the date-picker and time-picker editors to be hidden behind the modal, I think it's better to simply remove the z-index from the modal altogether.

#548 Is one way to resolve this issue (by removing the resize handles entirely).