Remove unused event listener types.

Remove ObjCEventListenerType and GObjectEventListenerType. They are unused
in Blink.

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/100703003

git-svn-id: svn://svn.chromium.org/blink/trunk@163059 bbb929c8-8fbe-4397-9dbb-9b2b20218538