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/ios/iosdeploystep.cpp

@@ -59,6 +59,7 @@ const Core::Id IosDeployStep::Id("Qt4ProjectManager.IosDeployStep");
 IosDeployStep::IosDeployStep(BuildStepList *parent)
     : BuildStep(parent, Id)
 {
+    setImmutable(true);
     updateDisplayNames();
     connect(DeviceManager::instance(), &DeviceManager::updated,
             this, &IosDeployStep::updateDisplayNames);