Datepicker Memory Leak

Building off #2268, there is a problem when destroying the datepicker where it doesn’t completely get removed from memory. This pull request, https://github.com/jquery/jquery-ui/commit/817ce38, removes the instance reference but the actual UI reference is tied to

https://github.com/jquery/jquery-ui/blob/54f96eea31b21d9ecb00912261df3e5aaebf8cce/ui/widgets/datepicker.js#L163

When the widget is destroyed, the datepicker UI element remains in memory preventing it from closing. https://github.com/jquery/jquery-ui/pull/2268 patches this issue by hiding the component, however dpDiv is still in memory. One solution could be to remove dpDiv from the instance and then set both _curInst = null and this.dpDiv = null.
https://github.com/jquery/jquery-ui/pull/2268/commits/5e4f1b86296c5a977abc7efaff671ea9004d4d37
However, many unit tests will break because they rely on dpDiv to still exist in memory, requiring the datepicker to be recreated for each test. There is also a problem that many developers rely on hiding the datepicker before destroying it $.datepicker("hide").destroy("destroy") which will fail because dpDiv will be null.

Inspect ui/widgets/datepicker.js around line 163 and trace the destroy path described in the issue. Review the unit tests that rely on dpDiv and reproduce the hide-then-destroy sequence. Done means the datepicker UI is no longer retained after destruction while the affected tests and documented usage behavior are addressed.