Remove a lot of dead code from AXNodeObject

Remove a lot of dead code from AXNodeObject. The following virtual function is
no longer used:
virtual void accessibilityText(Vector<AccessibilityText>&);

BUG=352771
R=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/202953005

git-svn-id: svn://svn.chromium.org/blink/trunk@169794 bbb929c8-8fbe-4397-9dbb-9b2b20218538