Remove the --no-threads workaround for wasm targets.

Remove `--no-threads` from the wasm-ld command-line, which was a
workaround for [an old bug] which was fixed in LLVM 9.0, and is
no longer needed.

Also, the `--no-threads` option has been [removed upstream].

[an old bug]: https://bugs.llvm.org/show_bug.cgi?id=41508
[removed upstream]: https://reviews.llvm.org/D76885

src/librustc_target/spec/wasm32_base.rs

@@ -10,13 +10,6 @@ pub fn options() -> TargetOptions {
         clang_args.push(format!("-Wl,{}", arg));
     };
 
-    // There have been reports in the wild (rustwasm/wasm-bindgen#119) of
-    // using threads causing weird hangs and bugs. Disable it entirely as
-    // this isn't yet the bottleneck of compilation at all anyway.
-    //
-    // FIXME: we should file an upstream issue with LLD about this
-    arg("--no-threads");
-
     // By default LLD only gives us one page of stack (64k) which is a
     // little small. Default to a larger stack closer to other PC platforms
     // (1MB) and users can always inject their own link-args to override this.