Lightbox close cleanup patch

When a Lightbox2 slideshow that displays captions is closed and then another slideshow is opened that has no captions a bug is seen. Lightbox does not clean up the Lightbox.imageArray array and subsequently the captions for the first slideshow are displayed on the second slideshow.

The attached patch adds two lines of code to the Lightbox.end function to ensure the captions are not seen a second time. It shouldn't affect any other operations, but is good practice.