Thanks for writing and for your question.
Currently there is not a straightforward way to remove this button. If you observe the behavior of the native Date/TimePicker controls, you will notice that they also have a Close button in their popups. Our controls are designed to follow the native Windows Phone OS behavior and therefore they also have such buttons. If it is urgent for you to remove this button, we will put this request on our TODO list for the upcoming SP2 release.
Have a suggestion or face a problem - you can use the Ideas & Feedback portal
to submit ideas, feedback and vote for them.