[libcalamares] Enforce singleton-ness of CalamaresPython::Helper

- unset instance pointer on destruction
 - make constructor private, and the instance accessor
   should create an instance if there isn't one.

src/libcalamares/JobQueue.cpp

@@ -41,9 +41,6 @@ public:
         , m_queue( queue )
         , m_jobIndex( 0 )
     {
-#ifdef WITH_PYTHON
-        new CalamaresPython::Helper( this );
-#endif
     }
 
     void setJobs( const JobList& jobs )