samples/shields: x_nucleo_iks01a3/02a1: Default use of prj.conf

By default, prj.conf is the CONF_FILE file use by any application.
Use the file when it is not the case and remove the line that sets
it in CMakeLists.txt as this is not required.

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