do not stderr that NoDisplay is set

Summary:
KF5PackageMacros turns stderr output into cmake level warnings.

setting nodisplay is perfectly reasonable and doesn't warrant a warning.
additionally these warnings are turned into test data by the CI, so
every plasmoid with nodisplay is causing unstable builds for no reason.

iff kpackagetool had a -o outputfile argument and stopped using stdout
for file output there'd be a place to print non-essential info such as
nodisplay being off. alas, no such luck.

Reviewers: mart, sebas, apol, dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6558