Remove redundant logic about showing navigation bar

PhoneWindowManager.layoutNavigationBar() has already made nav bar
show while status bar is expanded. The logic here is redundant.

Test: 1. go/wm-smoke
      2. Open a video in YouTube in fullscreen mode and then expand
         status bar, and see if navigation bar would show; collapse
	 status bar, and see if navigation bar would hide

Change-Id: I5327701ce1ad755354e42b6e14643f301abb6a5b