Remove unused archive flag from diff commands

* Option left in images/diff.go CLI as comment implies it is needed for
  backwards compatibility.

```release-note
NONE
```

[NO NEW TESTS NEEDED]

Signed-off-by: Jhon Honce <jhonce@redhat.com>

cmd/podman/diff.go

@@ -35,9 +35,6 @@ func init() {
 		Command: diffCmd,
 	})
 	flags := diffCmd.Flags()
-	// FIXME: Why does this exists? It is not used anywhere.
-	flags.BoolVar(&diffOpts.Archive, "archive", true, "Save the diff as a tar archive")
-	_ = flags.MarkHidden("archive")
 
 	formatFlagName := "format"
 	flags.StringVar(&diffOpts.Format, formatFlagName, "", "Change the output format (json)")