[1.2.X] Fixed #13390 -- `SplitDateTimeWidget` now recognizes when it's no...

[1.2.X] Fixed #13390 -- `SplitDateTimeWidget` now recognizes when it's no longer required.  Thanks vaxXxa for bug report and patch.

Backport of r13753 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13754 bcc190cf-cafb-0310-a4f2-bffc1f526a37