app_server: do not add screen_where twice to B_MOUSE_IDLE message

Message contents before this fix:
```
- B_MOUSE_IDLE:
    $target: B_PREFERRED_TOKEN
    screen_where:
      - !BPoint {x: 297, y: 280}
      - !BPoint {x: 297, y: 280}
```

Change-Id: If7ea26be13d2eacb5762b246d076db6d051dfe6c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/10654
Haiku-Format: Haiku-format Bot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>
Tested-by: Commit checker robot <[email protected]>

src/servers/app/EventDispatcher.cpp

@@ -1010,7 +1010,6 @@ EventDispatcher::_CursorLoop()
 		} else if (status == B_TIMED_OUT) {
 			mouseIdleSent = true;
 			BMessage* mouseIdle = new BMessage(B_MOUSE_IDLE);
-			mouseIdle->AddPoint("screen_where", fLastCursorPosition);
 			fStream->InsertEvent(mouseIdle);
 		}
 	}