Fix unused_imports warning #2328

Job	Run time
cargo +1.56 build	3m 0s
cargo minimal-versions build	41s
cargo test (ubuntu-latest)	5m 20s
cargo +stable build	1m 18s
cargo test (macos-latest)	6m 41s
cargo +1.36 build (futures-{core, io, sink})	3m 41s
cargo +beta build	1m 6s
cargo test (windows-latest)	8m 32s
cargo +nightly build	1m 11s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	5m 46s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	8m 52s
cargo bench	1m 19s
cargo clippy	41s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	10m 40s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	6m 5s
codegen	42s
cargo doc	32s
cargo hack check --feature-powerset	4m 25s
cargo miri test	13m 25s
cargo fmt	10s
cargo build --target thumbv7m-none-eabi	55s
cargo test -Z sanitizer=address	4m 3s
cargo build --target thumbv6m-none-eabi	45s
cargo test -Z sanitizer=memory	3m 12s
cargo test -Z sanitizer=thread	3m 40s
	1h 36m 42s

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