autotest: Remove screenshot test from WindowManagerHotkeys.

I'm making the window manager write logged-in screenshots to
/home/chronos/user/Downloads instead of
/home/chronos/user/Downloads/Screenshots.  I don't see much
value in continuing to test the screenshot hotkey here --
there's already another test for Ctrl-Alt-T.

BUG=chromium-os:16883
TEST=still passes

Change-Id: I0cc9d65da42bdc312ee7d0efe9d7b938b67facd2
Reviewed-on: http://gerrit.chromium.org/gerrit/3678
Reviewed-by: Daniel Erat <derat@chromium.org>
Tested-by: Daniel Erat <derat@chromium.org>
2 files changed