Remove lingering references to ImageOutputFormat #1332

Job	Run time
test_features	19s
test_features (default)	1m 45s
clippy	1m 31s
test_features (rayon)	27s
Fuzz targets (afl)	2m 10s
Fuzz targets (cargo-fuzz)	47s
test_features (avif)	1m 12s
test_features (bmp)	24s
public_private_dependencies	26s
test_features (dds)	21s
test_avif_decoding	1m 28s
test_features (exr)	41s
test_other_archs (powerpc-unknown-linux-gnu)	2m 33s
test_features (ff)	23s
test_other_archs (i686-unknown-linux-gnu)	1m 30s
test_toolchains (1.67.1)	41s
test_features (gif)	28s
test_features (hdr)	23s
test_toolchains (nightly)	1m 25s
test_toolchains (beta)	1m 19s
test_features (ico)	35s
test_features (jpeg)	23s
build_benchmarks	52s
test_features (png)	30s
cargo-deny	19s
test_features (pnm)	26s
rustfmt	14s
test_features (qoi)	27s
test_features (tga)	27s
test_features (tiff)	37s
test_features (webp)	40s
	25m 43s

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