You can use the report /DVD/QS_DELETE or transaction /DVD/QS_DEL for cleanup of extended query statistics that are collected in tables /DVD/QS_INFO and /DVD/QS_QVAR. 

Scheduling cleanup

If you want to cleanup statistics regularly, you need to create dynamic variant for the report /DVD/QS_DELETE as follows.

1. Open transaction SE38 and type report name
2. Click on button 
3. Chose new variant name and click on button 
4. Click on button 
5. Create variable for field Date
   1. Press F4 on Selection variable column for Date filed and choose witch date (System Date or Local Date) should be used for dynamic calculation and confirm

      
      

      
      
      
   2. Press F4 on Name of Variable column for Date filed and choose select option that should be used for execution. In our case we chose everything less or equal than current day minus ??? days and confirm

      
      

      
      
      
   3. Now choose how many days should be subtracted from current day and confirm
      
   4. Save variant
   5. Use created variant and schedule job that will be running on regular basis