remove redundant impl Unpin #2373

Job	Run time
cargo bench	1m 14s
cargo +stable build	39s
codegen	34s
cargo clippy	38s
cargo +1.36 build (futures-{core, io, sink})	2m 51s
cargo doc	20s
cargo hack check --feature-powerset	2m 40s
cargo +beta build	1m 9s
cargo fmt	6s
cargo +nightly build	45s
cargo minimal-versions build	32s
cargo miri test	6m 44s
cargo build --target thumbv7m-none-eabi	47s
cargo build --target thumbv6m-none-eabi	26s
cargo test -Z sanitizer=address	2m 46s
cargo +1.56 build	2m 4s
cargo test -Z sanitizer=memory	3m 39s
cargo test (ubuntu-latest)	2m 55s
cargo test -Z sanitizer=thread	2m 15s
cargo test (macos-latest)	6m 35s
cargo test (windows-latest)	4m 6s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 30s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	10m 5s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 43s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 17s
	1h 7m 20s

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