list_selection_model.js: Remove the debug output.

BUG=none
TEST=manually
TBR=yoshiki@chromium.org

Review URL: https://codereview.chromium.org/30693006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230361 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230361