ppp: unconditionally start pppd with nodefaultroute and usepeerdns (#12580)

ppp: unconditionally start pppd with nodefaultroute and usepeerdns (#12580)

Since the switch to netifd, proto handlers may always set the defaultroute
and provide dns server addresses, netifd will decide in the generic code
path whether the announced values are masked or not.

Additionally protocol handlers should not modify the routing tables themselves
and prevent any launched services from doing so.

Remove the additional defaultroute and peerdns option handling from the ppp.sh
protocol handler and rely on netifd to mask or not mask the values.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34536 3c298f89-4303-0410-b956-a3cf2f4a3e73