kernel-doc: do not regard $, %, or & prefixes as special in section names

The use of these is confusing in the script, and per this grep, they're
not used anywhere anyway:

$ git grep " \* [%$&][a-zA-Z0-9_]*:" -- *.[ch] | grep -v "\$\(Id\|Revision\|Date\)"

While at it, throw out the constants array, nothing is ever put there
again.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>