I found this way as a quick work around:
http://www.learningsharepoint.com/2013/05/18/hide-callouts-in-a-sharepoint-2013-task-list/
But by this solution, the drag-and-drop area is not rendered anymore and the old-fashioned "Add new item"-link below the list returns.
So I spent a little more time with googeling and found this page which adds an additional icon to click which displays the ECB directly:
http://www.booden.net/DirectAccessToEcb.aspx
The site describes the How-To and provides a sandboxed-solution for directly adding to the solutions-gallery of the SiteCollection. After activating everything to do is finished.
As you can see, the standard behaviour is not touched but extended by a new functionality:
It depends to the customer's wish if the three-dots-button now should be hidden (needs some additional work) or both buttons should remain.
