Remove a custom command for the system. If the argument does not
specify a valid custom command this call does nothing.
Built-in commands cannot be removed. (However they can be greyed
out in Softimage's Menus. See Command.Enabled.)
Note: This command does not delete the implementation of the
command. It simply removes the reference to the command from
Softimage's command list.
To remove a custom command that is defined as part of a
self-installed plug-in remove the script file or dll rather than
calling this method. Similarly, this method cannot be used to
remove custom commands that are installed as part of an add-on.