Refactor: Modernize disallowed copy constructors/assignment

Use C++11's better capability of expressing an interface of a non-copyable class by publicly deleting its copy ctor and assignment operator instead of just declaring them private.

src/net.h

@@ -702,13 +702,11 @@ public:
 
     CNode(NodeId id, ServiceFlags nLocalServicesIn, int nMyStartingHeightIn, SOCKET hSocketIn, const CAddress &addrIn, uint64_t nKeyedNetGroupIn, uint64_t nLocalHostNonceIn, const CAddress &addrBindIn, const std::string &addrNameIn = "", bool fInboundIn = false);
     ~CNode();
+    CNode(const CNode&) = delete;
+    CNode& operator=(const CNode&) = delete;
 
 private:
-    CNode(const CNode&);
-    void operator=(const CNode&);
     const NodeId id;
-
-
     const uint64_t nLocalHostNonce;
     // Services offered to this peer
     const ServiceFlags nLocalServices;