Remove GenericImage<Pixel = Rgba<u8>> impl for DynamicImage #1622

Job	Run time
build_benchmarks	43s
Fuzz targets (afl)	2m 4s
cargo-deny	18s
Fuzz targets (cargo-fuzz)	49s
cargo-semver-checks	23s
clippy	1m 59s
public_private_dependencies	25s
rustfmt	4s
test_avif_decoding	1m 20s
test_features	19s
test_other_archs (powerpc-unknown-linux-gnu)	2m 23s
test_toolchains (1.67.1)	45s
test_features (default)	1m 48s
test_other_archs (i686-unknown-linux-gnu)	1m 16s
test_toolchains (nightly)	1m 8s
test_features (rayon)	21s
test_toolchains (beta)	1m 17s
test_features (avif)	54s
test_features (bmp)	20s
test_features (dds)	33s
test_features (exr)	34s
test_features (ff)	20s
test_features (gif)	22s
test_features (hdr)	22s
test_features (ico)	26s
test_features (jpeg)	26s
test_features (png)	24s
test_features (pnm)	16s
test_features (qoi)	22s
test_features (tga)	21s
test_features (tiff)	27s
test_features (webp)	27s
	23m 56s

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