[red-knot] Inline Type::is_literal

[AlexWaygood]

The name of this method is somewhat confusing currently, as I would expect it to return true for LiteralString, but it doesn't (and LiteralString is displayed differently, so it wouldn't suit the method's current use case for it to include LiteralString). The method is only used in one location currently, so let's just inline it at that location.

[MichaReiser]

Thanks

[codspeed-hq]

CodSpeed Performance Report

Merging #13230 will improve performances by 4.66%

_{Comparing alex/is-literal (d9fd5d1) with main (50c8ee5)}

Summary

⚡ 1 improvements
✅ 31 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	alex/is-literal	Change
⚡	linter/default-rules[pydantic/types.py]	1.9 ms	1.8 ms	+4.66%

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.