numfmt: remove duplicate info from help output

[cakebaker]

clap automatically shows a list of possible values for an option in the help output, so it is unnecessary that we list the same values again. This PR removes this duplicate information.

// before the change
--round <METHOD>     use METHOD for rounding when scaling; METHOD can be: up,down, from-zero, towards-zero, nearest [default: from-zero] [possible values:
                     up, down, from-zero, towards-zero, nearest]

// after the change
--round <METHOD>     use METHOD for rounding when scaling [default: from-zero] [possible values: up, down, from-zero, towards-zero, nearest]