Remove caching.  Timings show that it is no longer a measurable improvement.  Worse, TSPAN collisions are causing mis-renderings.
Review URL: https://codereview.appspot.com/7645044

git-svn-id: http://blockly.googlecode.com/svn/trunk@696 c400ca83-b69d-9dd7-9705-49c6b8615e23