Remove the Clamped descriptor type

This is just a bit too general to work with and is pretty funky. Instead
just tweak `Clamped<&[u8]>` to naturally generate a descriptor for
`Ref(Slice(ClampedU8))`, requiring fewer gymnastics when interpreting
descriptors.

crates/cli-support/src/js/js2rust.rs

@@ -204,7 +204,7 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
                 i = i,
                 val = val,
             ));
-            if arg.is_by_ref() || arg.is_clamped_by_ref() {
+            if arg.is_by_ref() {
                 if optional {
                     bail!("optional slices aren't currently supported");
                 }