[red-knot] Minor simplifications to `mro.rs` #37177

Job	Run time
cargo fmt	22s
cargo build (release)	0s
Determine changes	8s
mkdocs	2m 40s
python package	1m 16s
pre-commit	36s
cargo clippy	1m 43s
benchmarks	5m 23s
cargo build (msrv)	3m 54s
cargo fuzz build	0s
cargo shear	22s
cargo test (linux)	1m 45s
cargo test (linux, release)	3m 38s
cargo test (wasm)	1m 42s
cargo test (windows)	3m 56s
test scripts	58s
formatter instabilities and black similarity	0s
fuzz parser	0s
ecosystem	2m 41s
test ruff-lsp	40s
	31m 44s

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