[network-data] remove unused enum constants (#6129)

src/core/thread/network_data.hpp

@@ -976,13 +976,6 @@ protected:
     uint8_t mLength;         ///< The number of valid bytes in @var mTlvs.
 
 private:
-    enum
-    {
-        kDataResubmitDelay  = 300000, ///< DATA_RESUBMIT_DELAY (milliseconds) if the device itself is the server.
-        kProxyResubmitDelay = 5000,   ///< Resubmit delay (milliseconds) if deregister as the child server proxy.
-
-    };
-
     class NetworkDataIterator
     {
     public: