Kconfig: Remove copy-pasted comments on some promptless symbols

Remove the

    # Omit prompt to signify a "hidden" option

comments that appear on some symbols. They seem to have been copy-pasted
at random, as there are lots of promptless symbols that don't have them
(that's confusing in itself, because it might give the idea that the
ones with comments are special in some way).

I suspect those comments wouldn't have helped me much if I didn't know
Kconfig either. There's a lot more Kconfig documentation now too, e.g.
https://docs.zephyrproject.org/latest/guides/kconfig/index.html

.

Keep some comments that give more information than the symbol having no
prompt.

Also do some minor drive-by cleanup.

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