remove redundant impl Unpin #2374

Job	Run time
codegen	26s
cargo +stable build	39s
cargo clippy	27s
cargo +beta build	43s
cargo bench	51s
cargo +nightly build	41s
cargo +1.36 build (futures-{core, io, sink})	3m 2s
cargo doc	33s
cargo fmt	8s
cargo hack check --feature-powerset	2m 40s
cargo minimal-versions build	26s
cargo miri test	7m 18s
cargo build --target thumbv7m-none-eabi	41s
cargo test -Z sanitizer=address	2m 52s
cargo build --target thumbv6m-none-eabi	29s
cargo test (ubuntu-latest)	2m 48s
cargo test -Z sanitizer=memory	3m 40s
cargo +1.56 build	2m 0s
cargo test (macos-latest)	4m 45s
cargo test -Z sanitizer=thread	2m 10s
cargo test (windows-latest)	4m 19s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 51s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	5m 59s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 53s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	5m 39s
	1h 4m 0s

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