Don't look up the main table any more.

After https://android-review.googlesource.com/#/c/481397/ ,
directly-connected routes for all network types are added to
the correct routing tables by ConnectivityService. So there
should be no reason to look up the main table.

Bug: 28825988
Test: bullhead builds, boots
Test: netd_{unit,integration}_test pass
Test: dual-stack wifi and IPv4-only mobile data work
Change-Id: I64ba7dbf71478afcd9d2880440f93ef346116b6b