Don't generate multi-return functions in wit-smith (#1699)

This fixes a fuzz issue from #1670 which wasn't fixed at the time and
has been found on oss-fuzz.

crates/wit-smith/src/generate.rs

@@ -846,9 +846,6 @@ impl<'a> InterfaceGenerator<'a> {
         dst.push_str("func");
         self.gen_params(u, dst, method)?;
         if u.arbitrary()? {
-            dst.push_str(" -> ");
-            self.gen_params(u, dst, false)?;
-        } else if u.arbitrary()? {
             dst.push_str(" -> ");
             let mut fuel = self.config.max_type_size;
             self.gen_type(u, &mut fuel, dst)?;