Drop explicit %{PROJECTDIR}/ from templates' ShowFilesAfterGeneration

Using the var with the ShowFilesAfterGeneration entry is not needed
starting with 5.1.1 (again), as it will be automatically prepended
for all relative paths, as defined in kdevplatform/plugins/appwizard/README.dox

Not using the var makes files easier to read & avoids typos
in the var name. So let default templates use recommended notation.