boards: st: stm32CubeProgrammer runner does not need hex-file path

Do not give the path to the zephyr.hex when flashing with the
STM32CubeProgrammer in MemoryMapped mode.
West flash can retrieve the path and file correctly. Twister cannot
find the build directory, the hex file is found correctly, though.

Signed-off-by: Francois Ramu <francois.ramu@st.com>