fix(derive): Don't generate dead code
fix(derive): Don't generate dead code

When debugging #2586, I noticed we were developing match cases for
variant names that were flattened.  At minimum, this is dead code and at
worst this could cause the wrong behavior if a user does an update with
one of those names.

Depends on #2587

Fixes #2588

clap-rs/clap,clap-rs/clap,clap-rs/clap