chromeos-events: Remove SystemKeyEventListener::ModifierObservers.

There are no ModifierObservers anymore. So remove them.

BUG=none
R=derat@chromium.org

Review URL: https://codereview.chromium.org/191523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255736 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255736