It is unused. Also remove singleProjectAdded signal, and replace
the only use with the projectAdded signal.

The singleProjectAdded signal was emitted for each project that was
ever added, simply because nobody ever used addProjects(...) with
more than one project.

Change-Id: I5aee315c64a2cfb721471d580989a6a02d44fefc
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>