Removes deprecated checkpoint functionality from GWTTestCase.

This was no-op before and last remaining references cleaned up with removal of Profile code.

Change-Id: I776d1fbef690d2375b25a8a58ea47cce943b26ec
Review-Link: https://gwt-review.googlesource.com/#/c/2001/

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11528 8db76d5a-ed1c-0410-87a9-c151d255dfc7