Remove dead code from Dictionary

Remove getEventListener() method from the Dictionary class as it does not
appear to be used and it always returns 0.

R=haraken
BUG=

Review URL: https://chromiumcodereview.appspot.com/24069003

git-svn-id: svn://svn.chromium.org/blink/trunk@157523 bbb929c8-8fbe-4397-9dbb-9b2b20218538