Remove the old x86 backend

tests/all/wast.rs

@@ -25,12 +25,6 @@ fn run_wast(wast: &str, strategy: Strategy, pooling: bool) -> anyhow::Result<()>
     // by reference types.
     let reftypes = simd || feature_found(wast, "reference-types");
 
-    // Threads & simd aren't implemented in the old backend, so skip those
-    // tests.
-    if (threads || simd) && cfg!(feature = "old-x86-backend") {
-        return Ok(());
-    }
-
     let mut cfg = Config::new();
     cfg.wasm_simd(simd)
         .wasm_bulk_memory(bulk_mem)