cmd: demo: Remove duplicated help message for list subcommand

There is no need to show demo list description twice when help demo is
performed. The patch removes duplicated entry.

Current state:
=> help demo
demo - Driver model (dm) demo operations

Usage:
demo list                     List available demo devices
demo hello <num> [<char>]     Say hello
demo light [<num>]            Set or get the lights
demo status <num>             Get demo device status
demo list                     List available demo devices

Fixes: a02af4ae ("dm: demo: Add a simple GPIO demonstration")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>