Remove `num-rational` #944

Job	Run time
Fuzz targets (afl)	2m 32s
Fuzz targets (cargo-fuzz)	6m 56s
build_benchmarks	1m 12s
cargo-deny	44s
clippy	3m 40s
public_private_dependencies	27s
rustfmt	21s
test_avif	1m 51s
test_big_endian	56s
test_big_endian (default)	1m 32s
test_big_endian (webp)	50s
test_big_endian (webp-encoder)	1m 33s
test_avif_decoding	2m 45s
test_features	33s
test_features (default)	2m 21s
test_features (gif)	45s
test_features (jpeg)	39s
test_features (png)	44s
test_features (tiff)	53s
test_features (ico)	46s
test_features (pnm)	44s
test_features (tga)	51s
test_features (webp)	38s
test_features (bmp)	45s
test_features (hdr)	37s
test_features (dxt)	44s
test_features (dds)	40s
test_features (farbfeld)	38s
test_features (openexr)	1m 12s
test_features (jpeg_rayon)	57s
test_features (webp-encoder)	1m 15s
test_toolchains (1.61.0)	31s
test_toolchains (nightly)	1m 44s
test_toolchains (beta)	1m 44s
verify_msrv	2m 28s
	47m 28s

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