small code improvements

src/ntp_client.cpp

@@ -37,7 +37,3 @@ void NtpClient::setDateTime() {
         sleep_ms(500);
     }
 }
-
-void NtpClient::stop() {
-    sntp_stop();
-}

src/ntp_client.h

@@ -8,9 +8,11 @@ extern "C" void set_system_time(u32_t sec);
 
 class NtpClient {
   public:
+    NtpClient() = delete;
     static void setDateTime();
     static void stop();
 
   private:
-    NtpClient() {} // Disallow creating an instance of this object
-};
+};
+
+inline void NtpClient::stop() { sntp_stop(); }