Filter off-by-default RUF014 out of schema (#5832)

**Summary** Previously, `RUF014` would be part of ruff.schema.json
depending on whether or not the `unreachable-code` feature was active.
This caused problems for contributors who got unrelated RUF014 changes
when updating the schema without the feature active.

An alternative would be to always add `RUF014`.

**Test plan** `cargo dev generate-all` and `cargo run --bin ruff_dev
--features unreachable-code -- generate-all` now have the same effect.

crates/ruff_dev/src/generate_json_schema.rs

@@ -61,7 +61,7 @@ mod tests {
 
     use super::{main, Args};
 
-    #[cfg_attr(not(feature = "unreachable-code"), test)]
+    #[test]
     fn test_generate_json_schema() -> Result<()> {
         let mode = if env::var("RUFF_UPDATE_SCHEMA").as_deref() == Ok("1") {
             Mode::Write