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/init.cpp

@@ -689,12 +689,6 @@ bool AppInit2(boost::thread_group& threadGroup)
                 SetLimited(net);
         }
     }
-#if defined(USE_IPV6)
-#if ! USE_IPV6
-    else
-        SetLimited(NET_IPV6);
-#endif
-#endif
 
     CService addrProxy;
     bool fProxy = false;
@@ -706,10 +700,8 @@ bool AppInit2(boost::thread_group& threadGroup)
         if (!IsLimited(NET_IPV4))
             SetProxy(NET_IPV4, addrProxy, nSocksVersion);
         if (nSocksVersion > 4) {
-#ifdef USE_IPV6
             if (!IsLimited(NET_IPV6))
                 SetProxy(NET_IPV6, addrProxy, nSocksVersion);
-#endif
             SetNameProxy(addrProxy, nSocksVersion);
         }
         fProxy = true;
@@ -751,9 +743,7 @@ bool AppInit2(boost::thread_group& threadGroup)
         else {
             struct in_addr inaddr_any;
             inaddr_any.s_addr = INADDR_ANY;
-#ifdef USE_IPV6
             fBound |= Bind(CService(in6addr_any, GetListenPort()), BF_NONE);
-#endif
             fBound |= Bind(CService(inaddr_any, GetListenPort()), !fBound ? BF_REPORT_ERROR : BF_NONE);
         }
         if (!fBound)