This reverts commit 9910ec700c.
## Summary
This change introduced a serious performance regression. Revert it while
we investigate.
Fixes https://github.com/astral-sh/ty/issues/431
## Test Plan
Timing on the snippet in https://github.com/astral-sh/ty/issues/431
again shows times similar to before the regression.
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/ty_test/README.md for documentation of this test format.