salhelper::SingletonRef copy ctor is broken, mark as deleted

If that (implicitly defined) ctor had ever been used, it would have failed to
increment m_nRef, so could have caused m_pInstance to be deleted too early.  So
better mark it as deleted.  (LO itself apparently didn't call it.  If 3rd party
code would want to call it after all, it could be changed from deleted to
properly user-provided, incrementing m_nRef, in the future.)

The implicitly defined copy assignment op happens to already do the right thing,
and /is/ used, e.g., during

>         m_aNode = _rhs.m_aNode;

in DriversConfig::operator= (connectivity/source/commontools/DriversConfig.cxx).
Mark it as defaulted to avoid -Wdeprecated-copy with GCC trunk towards GCC 9.

Change-Id: I527123d9de837c311d30a270feb09dc173d9f411
Reviewed-on: https://gerrit.libreoffice.org/56512
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/salhelper/singletonref.hxx

@@ -131,6 +131,9 @@ class SingletonRef
             // <- GLOBAL SAFE
         }
 
+#if defined LIBO_INTERNAL_ONLY
+        SingletonRef & operator =(SingletonRef const &) = default;
+#endif
 
         /** @short  Allows rSingle->someBodyOp().
          */
@@ -157,7 +160,7 @@ class SingletonRef
     // helper
 
     private:
-
+        SingletonRef(SingletonRef &) SAL_DELETED_FUNCTION;
 
         /** @short  creates an own mutex for guarding static contents.