HPCC-8763 Remove dkc option from dfuplus usage text

Remove obsolete dkc option from dfuplus usage text.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #3942 from richardkchapman/dkc-helptext

HPCC-8763 Remove dkc option from dfuplus usage text

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>