refactor: Allow partially used genned code
refactor: Allow partially used genned code

Wow, I'm having a hard time summarizing this for the summary.

When we code-gen Settings using `impl_settings`, we have no control over
whether one instance of a Settings will use some or all functions.  I
have a fix that removes the one `ArgSettings::unset`, causing warnings
for `ArgSettings` despite needing it for `AppSettings`.

So I'm making it less dependent on how each instantiation uses it.

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