[branding] Example banner graphic that matches the default branding.desc

- Bogus "CalaMinix" (maybe the example branding should be consistent
   with that) banner.
 - Turn off the (example) banner in the default branding.

src/branding/default/branding.desc

@@ -126,7 +126,7 @@ strings:
 #
 # These filenames can also use substitutions from os-release (see above).
 images:
-    productBanner:       "banner.png"
+    # productBanner:       "banner.png"
     productIcon:         "squid.png"
     productLogo:         "squid.png"
     productWelcome:      "languages.png"