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/qmakeprojectmanager/qmakemakestep.cpp

@@ -56,7 +56,7 @@ QmakeMakeStep::QmakeMakeStep(BuildStepList *bsl, Utils::Id id)
     : MakeStep(bsl, id)
 {
     if (bsl->id() == ProjectExplorer::Constants::BUILDSTEPS_CLEAN) {
-        setClean(true);
+        setIgnoreReturnValue(true);
         setUserArguments("clean");
     }
     supportDisablingForSubdirs();
@@ -89,11 +89,6 @@ bool QmakeMakeStep::init()
         workingDirectory = bc->buildDirectory();
     pp->setWorkingDirectory(workingDirectory);
 
-    // If we are cleaning, then make can fail with a error code, but that doesn't mean
-    // we should stop the clean queue
-    // That is mostly so that rebuild works on a already clean project
-    setIgnoreReturnValue(isClean());
-
     Utils::CommandLine makeCmd(makeExecutable);
 
     QmakeProjectManager::QmakeProFileNode *subProFile = bc->subNodeBuild();
@@ -176,6 +171,7 @@ bool QmakeMakeStep::init()
             qmakeStep->setForced(true);
     }
 
+    // Note: This skips the Makestep::init() level.
     return AbstractProcessStep::init();
 }