Normative: Make SharedArrayBuffer's sole parameter required (#2393)

SharedArrayBuffer.length is 1 on all web engines (as is
ArrayBuffer.length). Unsure why this was made optional.

eshost output for `print(SharedArrayBuffer.length)`:

```
1

1

1

1
```

spec.html

@@ -36412,8 +36412,8 @@ <h1>The SharedArrayBuffer Constructor</h1>
       </emu-note>
 
       <emu-clause id="sec-sharedarraybuffer-length">
-        <h1>SharedArrayBuffer ( [ _length_ ] )</h1>
-        <p>When the `SharedArrayBuffer` function is called with optional argument _length_, the following steps are taken:</p>
+        <h1>SharedArrayBuffer ( _length_ )</h1>
+        <p>When the `SharedArrayBuffer` function is called with argument _length_, the following steps are taken:</p>
         <emu-alg>
           1. If NewTarget is *undefined*, throw a *TypeError* exception.
           1. Let _byteLength_ be ? ToIndex(_length_).