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/qmakeprojectmanager/qmakestep.cpp

@@ -290,11 +290,6 @@ ProjectExplorer::BuildStepConfigWidget *QMakeStep::createConfigWidget()
     return new QMakeStepConfigWidget(this);
 }
 
-bool QMakeStep::immutable() const
-{
-    return false;
-}
-
 void QMakeStep::processStartupFailed()
 {
     m_needToRunQMake = true;