Tests

rm redundant return_type_to_pretty_string #1029

Job	Run time
build (macOS-latest, stable)	2m 52s
build (macOS-latest, nightly)	3m 12s
build (macOS-latest, 1.70)	3m 27s
build (ubuntu-latest, stable)	2m 21s
build (ubuntu-latest, nightly)	2m 12s
build (ubuntu-latest, 1.70)	3m 33s
build (windows-latest, stable)	7m 10s
build (windows-latest, nightly)	3m 40s
build (windows-latest, 1.70)	4m 44s
cargo-mutants	3h 26m 35s
	3h 59m 46s

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