lib: client reader polish

- seek_func/seek_client, use transfer values only
    - remove copies held in `struct connectdata`, use only
      ever `data->set.seek_func`
    - resolves possible issues in multiuse connections
    - new mime post reader eliminates need to ever overwriting this

- websockets, remove empty Curl_ws_done() function

Closes #13079

lib/vssh/wolfssh.c

@@ -625,10 +625,10 @@ static CURLcode wssh_statemach_act(struct Curl_easy *data, bool *block)
       if(data->state.resume_from > 0) {
         /* Let's read off the proper amount of bytes from the input. */
         int seekerr = CURL_SEEKFUNC_OK;
-        if(conn->seek_func) {
+        if(data->set.seek_func) {
           Curl_set_in_callback(data, true);
-          seekerr = conn->seek_func(conn->seek_client, data->state.resume_from,
-                                    SEEK_SET);
+          seekerr = data->set.seek_func(data->set.seek_client,
+                                        data->state.resume_from, SEEK_SET);
           Curl_set_in_callback(data, false);
         }