samples/drivers: led_apa102: Clean board specific Kconfig file

Various kconfig symbols where defined specifically for board
nucleo_l432kc. Though, most are generic to the sample so should moved
directly to sample prj.conf
Some others parts of boards default configuration so could be removed
as well.
Last remove CONFIG_OPENOCD_SUPPORT as sample has no dependency with
this symbol.

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