Merge `unused_tuple_struct_fields` into `dead_code` This implicitly upgrades the lint from `allow` to `warn` and places it into the `unused` lint group. [Discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Moving.20.60unused_tuple_struct_fields.60.20from.20allow.20to.20warn)