arch: stm32: remove .hex binary build by default

BUILD_OUTPUT_HEX was enabled by default for stm32 SoCs.
This should not be the default setting and besides it has no
effect because of 'default n' in misc/Kconfig that seems to
prevail.
Removing the 'default y' for  stm32 to avoid confusion.

Fixes #8193

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>