Drop the internal `rustc-rayon` impls #444

Job	Run time
clippy	17s
nostd_build (1.63.0, thumbv6m-none-eabi)	46s
miri	4m 28s
Check MSRV and minimal-versions	1m 1s
tests (1.63.0)	1m 22s
nostd_build (stable, thumbv6m-none-eabi)	7s
tests (stable, arbitrary)	34s
tests (stable, quickcheck)	35s
tests (stable, rayon)	47s
tests (stable, serde)	42s
tests (stable, borsh)	36s
tests (stable, std)	43s
tests (beta)	37s
tests (nightly, test build benchmarks)	36s
Success	1s
	13m 12s

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