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/autotoolsprojectmanager/autogenstep.cpp

@@ -138,8 +138,3 @@ BuildStepConfigWidget *AutogenStep::createConfigWidget()
 
     return widget;
 }
-
-bool AutogenStep::immutable() const
-{
-    return false;
-}