Remove build-time no-IPv6 setting

The year is 2014. All supported operating systems have IPv6 support, most certainly at build time (this doesn't mean that IPv6 is configured, of course).

If noone is exercising the functionality to disable it, that means it doesn't get tested, and IMO it's better to get rid of it.

(it's also not used consistently in RPC/boost and Net code: if USE_IPV6 is not enabled, it is still assumed that boost is compiled with ipv6 support, so this would never work...)