Remove https://streams.spec.whatwg.org/#ts-constructor step 12

[youennf]

Step 9 is: Let startPromise be a new promise.
Step 12 is: Let startPromise be a new promise.

Most probably step 12 should be removed.

[ricea]

Thanks. It was introduced accidentally in #1035. As you say, the second one should be removed. I will make a PR tomorrow.