[#975] Remove unused PostHeader widget.
[#975] Remove unused PostHeader widget.

Has dangling pointer to the template.

heiths/allura,Bitergia/allura,apache/incubator-allura,Bitergia/allura,leotrubach/sourceforge-allura,Bitergia/allura,apache/allura,lym/allura-git,lym/allura-git,lym/allura-git,apache/allura,leotrubach/sourceforge-allura,lym/allura-git,heiths/allura,apache/incubator-allura,lym/allura-git,heiths/allura,heiths/allura,Bitergia/allura,apache/allura,leotrubach/sourceforge-allura,Bitergia/allura,leotrubach/sourceforge-allura,apache/incubator-allura,apache/allura,heiths/allura,apache/allura,apache/incubator-allura