That's the only error that was ever checked for, in all nine
project manager. In the hypothetical case that we'll need something
else than the name of a file to identify a "project file", we'd
probably need to touch the signature anyway. Until then, remove
the duplication.

Change-Id: Iba00b8f71309a908e2d29c0a58c50b685eca0cae
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>