mbed_rtx.h not to include in platform

Author: deepikabhavnani

platform/mbed_retarget.cpp

@@ -42,7 +42,6 @@
 #include <stdio.h>
 #include <errno.h>
 #include "platform/mbed_retarget.h"
-#include "mbed_rtx.h"
 
 static SingletonPtr<PlatformMutex> _mutex;