Media: Clean up prior image edits if IMAGE_EDIT_OVERWRITE is true.
When IMAGE_EDIT_OVERWRITE is set to true, edited image files are
supposed to be deleted when an image is restored to the original.
However, when an image was edited more than once, and then restored,
files created during previous edits were left behind.
Fixes this behavior by updating wp_save_image() to clean up
leftover images after each edit when IMAGE_EDIT_OVERWRITE is true.
Props bradt, chriscct7, joemcgill.