Put the mouse out of selected objects
thenPress ESC ;) (On PC)

You have also if Menu Menu Select Opened / Press Deselect All

As Pilou said it, Escape key does it.

But if you want a shortcup key, you can use this code :
script:var gd = moi.geometryDatabase; gd.deselectAll();

Shutcut are defined in Options

Have fun

Thanks!