freertos-cmsis-rtos: Use user-provided FreeRTOSConfig.h

The freertos-cmsis-rtos CMake target included a configuration header
from the FreeRTOS-CMSIS repository. This was left over from some
experimentation when initially integrating the FreeRTOS-CMSIS. Remove
this include path and rely on the user-provided config header instead.

Signed-off-by: Robert Walton <rob.walton@arm.com>