Fixes a memory leak on IE with the cross-site fragment loading strategy.
Now the callbacks on script tags are removed when they are invoked.

Review at http://gwt-code-reviews.appspot.com/544801

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8200 8db76d5a-ed1c-0410-87a9-c151d255dfc7