followup: free_image: just check if a file in general exist

followup: free_image: just check if a file in general exist

don't require any specific file types, if something is here which can
be requested to delete over API/CLI it either comes from an API/CLI
operation and should be thus OK to delete, else a user caused the
creation of the special file and it either works and all is good or
the user gets notified as we check if unlink succeeded anyway

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>