helper/command: remove dead code in command_unknown()

helper/command: remove dead code in command_unknown()

Commit 89fa493a3bc34d22eeca06fa4e78523ac3b766a8 removes the top
level command "unknown" and adds in command_unknown() some unclear
code that should detect a user-implemented "unknown" command.
But, all the commands that trigger the execution of the function
command_unknown() are registered in register_command_handler() and
have the command name prefixed with "ocd_".
Due to such "ocd_" prefix in cmd_name, the condition
	if (strcmp(cmd_name, "unknown") == 0) { ... }
is always false.
Nobody complained for almost 10 years so we can proceed to remove
this dead code, before merging any change that obsoletes the
prefix "ocd_" and potentially makes the condition true.

Remove the dead code guarded by the condition above.

Change-Id: Icf11a956bb2d68fc84e6eb5779edf8e35db8fa53
Fixes: 89fa493a3bc3 ("remove unknown handler")
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5096
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>