Remove ash::switches::UseImmersiveFullscreenForAllWindows(). We are not considering turning the feature off. Remove its flag and get rid of some code complexity

BUG=None
TEST=None
R=oshima
TBR=jamescook, benwells for trivial removal of checking ash::switches::UseImmersiveFullscreenForAllWindows()

Review URL: https://codereview.chromium.org/199283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256952 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256952