Remove rebundant import of `std::convert::AsRef` #1259

Job	Run time
Test build-std (aarch64-tvos)	3m 8s
Test build-std (aarch64-tvos-sim)	3m 11s
Test (stable)	22s
Test CUDA support	38s
Test build-std (x86_64-tvos)	5m 17s
Test (beta)	1m 12s
Clippy	13s
Test (nightly)	1m 14s
MSRV (ubuntu-latest)	21s
Test (linux32)	1m 21s
Rustfmt	8s
MSRV (windows-latest)	45s
Test (macos)	1m 25s
Test (aarch64-macos)	41s
Test (aarch64-ios)	1m 1s
Test (win32)	1m 17s
Test (win64)	3m 11s
Test (mingw32)	1m 31s
Test (mingw64)	1m 37s
Test (windows-2019)	2m 15s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 13s
	33m 1s

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