Fix urls::errc reference

[alandefreitas]

errc is documented as:

#ifdef BOOST_URL_DOCS
enum errc
{
    __see_below__
};
#else
namespace errc = boost::system::errc;
#endif

It should always be namespace errc = boost::system::errc; and, since it's been deprecated for a few releases already, we must remove it from the index.