kconfig.py: Flag selects with unsatisfied deps and remove whitelisting

Turn the warning for selecting a symbol with unsatisfied dependencies
into an error. The last instance has been fixed (that triggers in CI at
least).

Also remove the warning whitelisting functionality, which was only used
to whitelist that warning. It hasn't been used for anything else in over
a year, so it probably wouldn't be useful to keep. Getting rid of it
makes the script easier to read.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>