nix-vector-routing: (fixes #373) Remove sub struct Ipv4AddressHash

`Ipv4NixVectorRouting::Ipv4AddressHash` should be removed
in favor of class `Ipv4AddressHash`. Struct `Ipv4AddressHash`
under Ipv4NixVectorRouting is redundant with class
`Ipv4AddressHash`.

Signed-off-by: Ameya Deshpande <ameyanrd@outlook.com>

src/nix-vector-routing/model/ipv4-nix-vector-routing.h

@@ -262,22 +262,6 @@ private:
   uint32_t m_totalNeighbors;
 
 
-  /**
-   * \brief Hashing for the ipv4Address class
-   */
-  struct Ipv4AddressHash
-  {
-    /**
-     * \brief operator ()
-     * \param address the IPv4 address to hash
-     * \return the hash of the address
-     */
-    size_t operator() (const Ipv4Address &address) const
-    {
-      return std::hash<uint32_t>()(address.Get ());
-    }
-  };
-
   /**
    * Mapping of IPv4 address to ns-3 node.
    *