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/android/androidpackageinstallationstep.cpp

@@ -53,6 +53,7 @@ AndroidPackageInstallationStep::AndroidPackageInstallationStep(BuildStepList *bs
     setDefaultDisplayName(name);
     setDisplayName(name);
     setWidgetExpandedByDefault(false);
+    setImmutable(true);
 }
 
 bool AndroidPackageInstallationStep::init(QList<const BuildStep *> &earlierSteps)
@@ -115,11 +116,6 @@ BuildStepConfigWidget *AndroidPackageInstallationStep::createConfigWidget()
 }
 
 
-bool AndroidPackageInstallationStep::immutable() const
-{
-    return true;
-}
-
 //
 // AndroidPackageInstallationStepWidget
 //