Revert "Allow modifying class names from header rows in StackPanel."

This reverts commit 5da4865d255d9c73bc44b5c3e875aa084dcdfe3e.

(Broke the build - http://build.gwtproject.org/job/gwt/220/testReport/)

Change-Id: If183d236f8fd5b51611063f2f67eb12af3864494