fix #4612: mobile: avoid crash due to missing getProxy method

fix #4612: mobile: avoid crash due to missing getProxy method

A previous commit (51083ee) added an override of the internal ExtJS
method `Ext.data.Store.onProxyLoad` in order to fix a race condition.
This override calls `Operation.getProxy`. However, this method is not
available on Sencha Touch. As a consequence, the mobile UI was
broken.

This commit modifies the override such that it has no effect in the
mobile UI.

Fixes: 51083ee54aa98af5a711622e4ed240840dcbbabe
Suggested-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Friedrich Weber <f.weber@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Dominik Csapak <d.csapak@proxmox.com>