Thumbnails and converted videos not properly deleted. Patch.

The thumbnails and converted videos are not properly deleted.

In video_field_delete(), the original is deleted first -- which means the lookups of thumbnails and converted videos don't return proper data, because the original record is gone. This patch simply rearranges the order of deletions so that the original is deleted last, allowing the thumbnails and converted videos to be found and deleted properly.

Note that even with this fix, any folders that were created for the converted videos and thumbnails are left behind, even after they are emptied out. We may need to deal with those as a separate issue, or decide if it's ok just to leave them.