RPC_CondCabling: Fix clang warning.

RPC_CondCabling: Fix clang warning.

RDOindex::operator= cannot be defaulted due to the presence of a 
reference member.

MuonSpectrometer/MuonConditions/MuonCondCabling/RPC_CondCabling/RPC_CondCabling/RDOindex.h

@@ -116,7 +116,7 @@ public:
     RDOindex(const RDOindex&) = default;
     ~RDOindex() = default;
 
-    RDOindex& operator=(const RDOindex&) = default;
+    RDOindex& operator=(const RDOindex&) = delete;
     operator bool() const;
     bool operator!() const;