Removed is_connected function

- Not supported by TCP/UDP protocols
- Uncommon and less useful with proper error handling

TCPSocket.cpp

@@ -50,11 +50,6 @@ int TCPSocket::connect(const char *host, uint16_t port)
     return connect(addr);
 }
 
-bool TCPSocket::is_connected()
-{
-    return _socket && _iface->socket_is_connected(_socket);
-}
-
 int TCPSocket::send(const void *data, unsigned size)
 {
     mbed::Timer timer;

TCPSocket.h

@@ -48,11 +48,6 @@ public:
      */
     int connect(const SocketAddress &address);
     
-    /** Check if the socket is connected
-     *  @return         true if connected, false otherwise
-     */
-    bool is_connected();
-    
     /** Send data to the remote host
      *  @param data     The buffer to send to the host
      *  @param size     The length of the buffer to send