Delete formatted images after image upload

This change fixes an issue where old/existing formatted images are used
when a filename is reused (by overwrite or by coincidence), regardless
of if the file contents have changed. To users this mainly manifests
as a file overwrite appearing not to work; the thumbnails in the CMS
show the original image until regeneration is forced.

Calling Image::deleteFormattedImages() after image upload ensures that
no stagnant formatted images will be used.