luci-app-mwan3: Remove unnecessary dependency

There is no need to depend on luci-app-firewall, so remove the dependency.

I have examined the code and cannot see any reason for the dependency
and I have built a router with luci-app-mwan3, and without firewall,
luci-app-firewall and it all functions just fine.

Signed-off-by: Brian J. Murrell <brian@interlinx.bc.ca>