This basically reverts commit 31fe84e4 "core: Add host route for
DHCP4 server if outside assigned subnet (bgo #721767)" because the
additional route added by NM does more harm than good.

First, the code does not consider routes pushed by the server and thus
it can add a route conflicting with the ones from the network
administrator.

Second, there is no specification on what a DHCP client should do when
the server is not reachable via unicast, and adding arbitrary logic
into the client is likely to break things in specific cases. If
network administrators want to make the DHCP server reachable from a
client in a different subnet, they should push proper routes with the
lease.

In any case, if the DHCP server is not reachable through unicast,
before the lease expiration (after timeout T2) the client will resort
to broadcast and so there won't be any network disruption; the renewal
will only happen at a later time.

Fixes: 31fe84e4

https://bugzilla.redhat.com/show_bug.cgi?id=1448987