cmd: cyclic: Remove duplicate command name in help text

Function 'cmd_usage()' already prints one command in usage before
printing out the help text given to the U_BOOT_CMD_WITH_SUBCMDS macro.

Wrong previous output:

    Usage:
    cyclic cyclic demo <cycletime_ms> <delay_us> - register cyclic demo function
    cyclic list - list cyclic functions

Signed-off-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Stefan Roese <sr@denx.de>