The command you want is
 OVERLAYCLOSEALL;
You can find commands by using the graphical user interface, in this case you would select the Insert/Overlays/OverlayCloseAll function. Whenever you select a command from the insert menu, a description of the command appears in the bottom of the scripting menu.

By the way, you could also use the 
 RESETDEFAULTS;
command, but this would close the overlays, the background and any other custom setting you chose. So the OverlayCloseAll function is the most specific for your needs. 

The scripting window's "File" menu will show you a series of typical scripts, that illustrate a lot of useful scripting functions. For example, the "new_2017" script shows some of new features added into the latest release.

-c