nautilus-canvas-dnd.c: destroy drag_info.selection_cache on drag_end_callback()

When the user is dragging items between locations, a drag_info.selection_cache is initialized within drag_begin_callback(). However, unlike other caches, the drag_info.selection_cache is never destroyed by drag_end_callback() once the drag operation is complete.

This patch frees the drag_info.selection_cache once the user drag action is complete.

Fixes #787 (closed)