luci-app-mwan3: remove direct configuration possibility for /etc/config files

With this change it is not longer possible to edit the following
configuration files directly.

- mwan3
- wireless
- network

The user should edit the configuration over the provided luci pages.
which will do input checks and so prevent from miss configuration.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>