FuturesUnordered: Fix clear implementation #2409

Job	Run time
cargo minimal-versions build	28s
cargo +1.36 build (futures-{core, io, sink})	30s
cargo +stable build	45s
cargo bench	50s
cargo +beta build	41s
cargo build --target thumbv7m-none-eabi	28s
cargo +nightly build	48s
cargo build --target thumbv6m-none-eabi	31s
cargo +1.56 build	1m 2s
cargo test (ubuntu-latest)	3m 45s
cargo test (macos-latest)	7m 46s
cargo clippy	26s
codegen	27s
cargo test (windows-latest)	6m 4s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	15m 47s
cargo hack check --feature-powerset	3m 2s
cargo fmt	12s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	23m 34s
cargo miri test	23m 46s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	23m 48s
cargo test -Z sanitizer=address	10m 8s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	4m 8s
cargo test -Z sanitizer=memory	4m 59s
cargo test -Z sanitizer=thread	3m 49s
	2h 17m 44s

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