Remove obsolete OS_IDLE_THREAD_STACK_SIZE from NRF52

Custom size is the same as the default size anyway.

Author: Marcus Chang

targets/TARGET_NORDIC/mbed_rtx.h

@@ -37,17 +37,12 @@
 #define INITIAL_SP              (0x20010000UL)
 #endif
 
-#define OS_IDLE_THREAD_STACK_SIZE  512
-
 #elif defined(TARGET_MCU_NRF52840)
 
 #ifndef INITIAL_SP
 #define INITIAL_SP              (0x20040000UL)
 #endif
 
-// More than 256 bytes are needed for the idle thread stack on the NRF52840
-#define OS_IDLE_THREAD_STACK_SIZE  512
-
 #endif // defined(TARGET_MCU_NRF51822)...
 
 #endif // #ifndef MBED_MBED_RTX_H