ProjectExplorer: Settle on fixed set of build step lists

Even if the implementation allowed different lists than 'build'
and 'clean', that's the only set that has been used, ever.

If really needed, this could be re-instated, but for now having
them fixed removes part of the neeed for a two-phase construction
and helps to simplify user code.

Change-Id: I3df09a1829a7d020ef8963d358ea80f8d199ba13
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/compilationdatabaseprojectmanager/compilationdatabaseproject.h

@@ -100,6 +100,7 @@ class CompilationDatabaseBuildConfiguration : public ProjectExplorer::BuildConfi
     Q_OBJECT
 public:
     CompilationDatabaseBuildConfiguration(ProjectExplorer::Target *target, Core::Id id);
+    void initialize() final {}
 };
 
 class CompilationDatabaseBuildConfigurationFactory