kconfig: Remove '# Hidden' comments on promptless symbols

How prompts work is better documented nowadays, and these comments might
not be that helpful if you don't know.

There are lots promptless symbols that don't have a comment.

Also fix up some comments in arch/Kconfig that seem misplaced/redundant,
and clean up some whitespace (no blank line after a comment makes it
look like it only applies to the symbol directly after it to me).

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