luci-base: remove interface from zone upon deletion

Fixes: #2690
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/luasrc/model/network.lua

@@ -622,6 +622,12 @@ function del_network(self, n)
 					_uci:delete("wireless", s['.name'], "network")
 				end
 			end)
+
+		local ok, fw = pcall(require, "luci.model.firewall")
+		if ok then
+			fw.init()
+			fw:del_network(n)
+		end
 	end
 	return r
 end