Remove troublesome last-ditch attempt to get IP address prefix

When running NetworkInterface::map compiled for Windows XP on newer version of Windows, the assert on line 1182 (now 1179) is often tripped because the "last-ditch" attempt to get the IP address prefix assumes that the first prefix in the interface list is the one we need. Unfortunately, the first prefix can sometimes correspond to an IPv6 address with a prefix length of 64 bits, causing the assertion to fail. This occurs consistently when attempting to enumerate disabled interfaces in addition to enabled ones.