ProjectExplorer: Remove redundant MakeStep::{is,set}Clean()

This was only used for setIgnoreReturnValue(), which can be done
directly and also does not need to be stored in the settings.

Change-Id: I7419efe66d164b223463e19c3daf2350b4a2bae4
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/genericprojectmanager/genericmakestep.cpp

@@ -47,7 +47,7 @@ GenericMakeStep::GenericMakeStep(BuildStepList *parent, Utils::Id id)
         setBuildTarget("all");
     } else if (parent->id() == ProjectExplorer::Constants::BUILDSTEPS_CLEAN) {
         setBuildTarget("clean");
-        setClean(true);
+        setIgnoreReturnValue(true);
     }
     setAvailableBuildTargets({"all", "clean"});
 }