rust: Fix unused_imports

rr_frontend/.cargo/config.toml

rr_frontend/attribute_parse/src/parse.rs

rr_frontend/translation/src/arg_folder.rs

rr_frontend/translation/src/base.rs

...rontend/translation/src/environment/dump_borrowck_info.rs

rr_frontend/translation/src/environment/mod.rs

rr_frontend/translation/src/environment/polonius_info.rs

rr_frontend/translation/src/traits.rs