read_callback: move to SessionHandle from connectdata

With many easy handles using the same connection for multiplexing, it is
important we store and keep the transfer-oriented stuff in the
SessionHandle so that callbacks and callback data work fine even when
many easy handles share the same physical connection.

lib/file.c

@@ -315,8 +315,6 @@ static CURLcode file_upload(struct connectdata *conn)
    * Since FILE: doesn't do the full init, we need to provide some extra
    * assignments here.
    */
-  conn->fread_func = data->set.fread_func;
-  conn->fread_in = data->set.in;
   conn->data->req.upload_fromhere = buf;
 
   if(!dir)