CMake: Use new AbstractProcessStep convenience functions

This is actually a slight change in behavior insofar as
the summary now uses the same environment as the build
itself. This should not matter, or even be more correct.

Change-Id: I48e5be6cb7b97606f80f563ba399c4b6ff61c3bf
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.h

@@ -96,7 +96,6 @@ private:
 
     QMetaObject::Connection m_runTrigger;
 
-    QString m_ninjaProgressString;
     QStringList m_buildTargets;
     QString m_cmakeArguments;
     QString m_toolArguments;