NetworkManager manages the 'resolv.conf' file globally, so there is no
need for pppd specific scripts for DNS.
These scripts race with NM to set DNS, leading sometimes to a scenario
where you have ethernet as the default gateway, but with DNS only
reachable via cellular.
While on it, also remove not used 'mm_cellular' configuration file.
Signed-off-by: Javier Viguera <javier.viguera@digi.com>