You should only get a default route on the ethernet if you have asked
for one - either by setting defaultroute in rc.conf or by having /etc/mygate
containing the default route gateway.

Delete whichever of those exists and the default route will go away, from
the setup you described, you don't need it (on the system where you run pppd,
on the other one, assuming you want to route through the ppp system, you
will want a default route via the ethernet).

kre