remove explicit "firewall" and "urngd" packages

... as recommended at
https://forum.freifunk.net/t/fehler-file-system-partition-too-big/24139/3 .

I am not aware that we explicitly want these packages; so they should only
be pulled in as dependencies from other packages, if necessary.

image-customization.lua

@@ -25,8 +25,6 @@ features({
 packages({
   'respondd-module-airtime',
   'iwinfo',
-  'firewall',
-  'urngd',
 })
 
 if not device_class('tiny') then