portable/GCC/ARM_CM3/port.c: tentative "official" fix for priority issues

See
https://sourceforge.net/projects/freertos/forums/forum/382005/topic/8550890
WM09/demo.c (main): we don't need to call NVIC_SetPriorityGrouping

The call to NVIC_PriorityGroupConfig already does what we need.

NVIC_PriorityGroupConfig comes from CMSIS while NVIC_SetPriorityGrouping
is STM's variation of the same theme.