Remove dead code and doc regarding the old SocksSocket option

In entry_connection_t, there is still is_socks_socket option that is simply not used.

Other place is this log which should be renamed to mention "Unix socket".

    log_info(LD_NET, "Closing SOCKS SocksSocket connection");

Finally, the man page is still listing the option within another option (although SocksSocketsGroupWritable is still a thing that we should probably rename).

       SocksSocketsGroupWritable 0|1
     If this option is set to 0, don’t allow the filesystem group to read and write unix sockets (e.g. SocksSocket). If the option
     is set to 1, make the SocksSocket socket readable and writable by the default GID. (Default: 0)