refactor: clean up errors.rs and error_codes_check.rs

Move them into new `error_codes.rs` tidy check.

Author: Ezrashaw

compiler/rustc_error_codes/src/error_codes.rs

@@ -443,7 +443,6 @@ E0725: include_str!("./error_codes/E0725.md"),
 E0726: include_str!("./error_codes/E0726.md"),
 E0727: include_str!("./error_codes/E0727.md"),
 E0728: include_str!("./error_codes/E0728.md"),
-E0729: include_str!("./error_codes/E0729.md"),
 E0730: include_str!("./error_codes/E0730.md"),
 E0731: include_str!("./error_codes/E0731.md"),
 E0732: include_str!("./error_codes/E0732.md"),