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/iosbuildstep.cpp

@@ -178,11 +178,6 @@ BuildStepConfigWidget *IosBuildStep::createConfigWidget()
     return new IosBuildStepConfigWidget(this);
 }
 
-bool IosBuildStep::immutable() const
-{
-    return false;
-}
-
 void IosBuildStep::setBaseArguments(const QStringList &args)
 {
     m_baseBuildArguments = args;