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/url.c

@@ -5844,8 +5844,6 @@ static CURLcode create_conn(struct SessionHandle *data,
    * Inherit the proper values from the urldata struct AFTER we have arranged
    * the persistent connection stuff
    */
-  conn->fread_func = data->set.fread_func;
-  conn->fread_in = data->set.in;
   conn->seek_func = data->set.seek_func;
   conn->seek_client = data->set.seek_client;