luci-app-firewall: fix cbi form for rules

 - Set src/dest defaults only in initial section create state, otherwise it
   is impossible to specify output rules
 - Get rid of dest_remote/dest_local widget switching and implement change
   logic directly in tools.widgets.CBIZoneSelect
 - Remove leftover debug code

Ref: https://github.com/openwrt/luci/issues/2889
Signed-off-by: Jo-Philipp Wich <jo@mein.io>