Fix unused_imports warning #2327

Job	Run time
cargo bench	1m 26s
cargo +stable build	1m 26s
cargo clippy	36s
codegen	41s
cargo +beta build	1m 5s
cargo +1.36 build (futures-{core, io, sink})	3m 23s
cargo doc	32s
cargo +nightly build	1m 15s
cargo hack check --feature-powerset	4m 8s
cargo fmt	8s
cargo minimal-versions build	35s
cargo miri test	14m 20s
cargo build --target thumbv7m-none-eabi	44s
cargo build --target thumbv6m-none-eabi	47s
cargo test -Z sanitizer=address	4m 59s
cargo test -Z sanitizer=memory	4m 37s
cargo test (ubuntu-latest)	4m 53s
cargo +1.56 build	2m 34s
cargo test -Z sanitizer=thread	3m 7s
cargo test (macos-latest)	7m 0s
cargo test (windows-latest)	8m 1s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	5m 38s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	8m 11s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	9m 1s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	4m 38s
	1h 33m 45s

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