Remove unnecessary or nonexistent files from Nping Windows build.

These were causing Nping to always appear to be out of date when
debugging. Cf. r26250 and r25068.
Enforce device name only after choosing loopback in route_dst_generic.

Nping calls route_dst at least twice: once with a NULL device, and again
with the device learned from the first time. This interfered with the
code that automatically chooses a loopback interface for dests that are
the same as an interface address. For example, if you are 192.168.0.1,
and you are scanning 192.168.0.1, route_dst will tell you to use
interface "lo0" even though the interface of 192.168.0.1 may be "eth0".
route_dst was returning failure because the device name check was being
done before "eth0" got changed to "lo0".

This problem didn't show up on Linux because Linux uses
route_dst_netlink, which delegates the work to the kernel. But I could
reproduce on Linux by forcing the use of route_dst_generic.