Remove the two default role names listed in cppmodels documentation.

The list is incomplete, and they are now fully documented under
QAbstractItemModel::roleNames(). This saves us maintaining two lists
that can easily get out of sync.

Change-Id: I4895a8ba19a4f48c26b4077e8bf2eba8c4c8407c
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>