kernel: Update FreeRTOS kernel to V10.6.1

Since we are integrating latest version of FreeRTOS TCP/IP stack which
relies on FreeRTOS kernel, ensure that we are using the latest released
version of FreeRTOS kernel which is `V10.6.1`.

In addtion, fix a typo and remove unused macro from `FreeRTOSConfig.h`.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>