Remove pointless Display.clear()

It served no meaningful purpose and it had bugs. So let's remove it
rather than try to fix it.

kasmweb/docs/API-internal.md

@@ -99,7 +99,6 @@ None
 | absY               | (y)                                                     | Return Y relative to the remote display
 | resize             | (width, height)                                         | Set width and height
 | flip               | (from_queue)                                            | Update the visible canvas with the contents of the rendering canvas
-| clear              | ()                                                      | Clear the display
 | pending            | ()                                                      | Check if there are waiting items in the render queue
 | flush              | ()                                                      | Resume processing the render queue unless it's empty
 | fillRect           | (x, y, width, height, color, from_queue)                | Draw a filled in rectangle