cleanup: move bogon checking code in netxlite (#562)

I develop this diff while working on https://github.com/ooni/probe/issues/1803#issuecomment-957323297.

While there, make sure we don't have duplicate bogon code
and always use the code inside netxlite.

internal/measurex/resolver.go

@@ -175,7 +175,7 @@ func (r *resolverDB) computeOddityLookupHost(addrs []string, err error) Oddity {
 		}
 	}
 	for _, addr := range addrs {
-		if isBogon(addr) {
+		if netxlite.IsBogon(addr) {
 			return OddityDNSLookupBogon
 		}
 	}