[NRF52840]: Removed unwanted nRF5 SDK's app_timer libarary.

The Library is unused by the port. It breaks "non-rtos" compilation.

Author: nvlsianpu

targets/TARGET_NORDIC/TARGET_NRF5_SDK13/TARGET_MCU_NRF52840/sdk/sdk_config.h

@@ -2883,7 +2883,7 @@
 // <e> APP_TIMER_ENABLED - app_timer - Application timer functionality
 //==========================================================
 #ifndef APP_TIMER_ENABLED
-#define APP_TIMER_ENABLED 1
+#define APP_TIMER_ENABLED 0
 #endif
 #if  APP_TIMER_ENABLED
 // <q> APP_TIMER_WITH_PROFILER  - Enable app_timer profiling