ProjectExplorer: Make BuildStep::immutable use a data member

Default to false, remove all no-op reimplementations.

And rename the getter to isImmutable according to the rules.

Change-Id: I8cce79d88fb59badfa1cffcf30a46f7ff3b09e8b
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.h

@@ -60,7 +60,6 @@ public:
     void run(QFutureInterface<bool> &fi) override;
 
     ProjectExplorer::BuildStepConfigWidget *createConfigWidget() override;
-    bool immutable() const override;
 
     QString buildTarget() const;
     bool buildsBuildTarget(const QString &target) const;