ZJIT: Remove dead unnecessary_transmutes allow

```
warning: unknown lint: `unnecessary_transmutes`
   --> zjit/src/cruby.rs:107:9
    |
107 | #[allow(unnecessary_transmutes)] // rust-lang/rust-bindgen#2807
    |         ^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unknown_lints)]` on by default
```

Author: st0012

zjit/src/cruby.rs

@@ -104,7 +104,6 @@ pub type RedefinitionFlag = u32;
 
 #[allow(unsafe_op_in_unsafe_fn)]
 #[allow(dead_code)]
-#[allow(unnecessary_transmutes)] // https://github.com/rust-lang/rust-bindgen/issues/2807
 #[allow(clippy::all)] // warning meant to help with reading; not useful for generated code
 mod autogened {
     use super::*;