scripts: remove CONFIG_IS_ENABLED and CONFIG_VAL in config_whitelist.txt

The helper macro CONFIG_IS_ENABLED and CONFIG_VAL are not real
configurations and they are no more present in u-boot.cfg so they can
be removed in config_whitelist.txt.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Simon Glass <sjg@chromium.org>