Change back default size of timer thread stack

Configuration for changing this size is kept.

Author: ccli8

rtos/TARGET_CORTEX/mbed_rtx_conf.h

@@ -35,12 +35,7 @@
 #define OS_STACK_SIZE               MBED_CONF_APP_THREAD_STACK_SIZE
 
 #ifndef OS_TIMER_THREAD_STACK_SIZE
-    #if (defined(__ARM_FEATURE_CMSE) && (__ARM_FEATURE_CMSE == 3U)) ||\
-        (defined(__DOMAIN_NS) && (__DOMAIN_NS == 1U))
-        #define OS_TIMER_THREAD_STACK_SIZE  1280
-    #else
-        #define OS_TIMER_THREAD_STACK_SIZE  768
-    #endif
+#define OS_TIMER_THREAD_STACK_SIZE  768
 #endif
 
 #ifndef OS_IDLE_THREAD_STACK_SIZE