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...)

src/netbase.h

@@ -77,10 +77,8 @@ class CNetAddr
         int GetReachabilityFrom(const CNetAddr *paddrPartner = NULL) const;
         void print() const;
 
-#ifdef USE_IPV6
         CNetAddr(const struct in6_addr& pipv6Addr);
         bool GetIn6Addr(struct in6_addr* pipv6Addr) const;
-#endif
 
         friend bool operator==(const CNetAddr& a, const CNetAddr& b);
         friend bool operator!=(const CNetAddr& a, const CNetAddr& b);
@@ -121,10 +119,8 @@ class CService : public CNetAddr
         std::string ToStringIPPort() const;
         void print() const;
 
-#ifdef USE_IPV6
         CService(const struct in6_addr& ipv6Addr, unsigned short port);
         CService(const struct sockaddr_in6& addr);
-#endif
 
         IMPLEMENT_SERIALIZE
             (