ProjectExplorer: Simplify BuildStep::init() signature

The extra parameter was always computed but used only in one place,
and that use got removed lately.

Change-Id: Ie10c0107ca70ee97ce03f83294992aab8d1a3ffe
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qbsprojectmanager/qbscleanstep.cpp

@@ -64,9 +64,8 @@ QbsCleanStep::~QbsCleanStep()
     }
 }
 
-bool QbsCleanStep::init(QList<const BuildStep *> &earlierSteps)
+bool QbsCleanStep::init()
 {
-    Q_UNUSED(earlierSteps);
     if (project()->isParsing() || m_job)
         return false;