Remove TODO from step 4 of delete

The confusion around step 4 of `delete` method has been resolved through https://github.com/whatwg/fetch/issues/372.

components/script/dom/headers.rs

@@ -105,8 +105,6 @@ impl HeadersMethods for Headers {
             return Ok(());
         }
         // Step 4
-        // TODO: Requires clarification from the Fetch spec:
-        // ... https://github.com/whatwg/fetch/issues/372
         if self.guard.get() == Guard::RequestNoCors &&
             !is_cors_safelisted_request_header(&valid_name, &b"invalid".to_vec()) {
                 return Ok(());