remove Rule 3 from `ref_pat_eat_one_layer_2024`

The debug assertion ensuring that the pattern mutability cap holds
assumes the presence of Rule 3, so it now checks for that. I
considered going back to only tracking the mutability cap when Rule 3
is present, but since the mutability cap is used in Rule 5's
implementation too, the debug assertion would still need to check
which typing rules are present.

This also required some changes to tests:
- `ref_pat_eat_one_layer_2021.rs` had a test for Rule 3; I'll be
handling tests for earlier editions in a later commit, so as a stopgap
I've #[cfg]ed it out.
- One test case had to be moved from `well-typed-edition-2024.rs` to
`borrowck-errors.rs` in order to get borrowck to run on it and emit an
error.

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