drop the setting of c++0x

Summary: this is already done by KDEFrameworkCompilerSettings so it's just duplicated now

Test Plan: still builds and runs

Reviewers: Plasma, kossebau

Reviewed By: kossebau

Subscribers: kossebau, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D10748