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/androidbuildapkstep.cpp

@@ -129,6 +129,7 @@ AndroidBuildApkStep::AndroidBuildApkStep(BuildStepList *parent)
 {
     //: AndroidBuildApkStep default display name
     setDefaultDisplayName(tr("Build Android APK"));
+    setImmutable(true);
 }
 
 bool AndroidBuildApkStep::init(QList<const BuildStep *> &earlierSteps)