helper/command: remove unused functions and make static local ones

helper/command: remove unused functions and make static local ones

The function command_find_in_parent() is never used in OpenOCD, so
remove it.

The functions command_name() and [un]register_command() are only
used internally in command.c, so make them static.

Change-Id: Ide9842659796f4884fb6c1fcf5979b3b71b67abb
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5663
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins