[heycam/webidl] Remove section numbers from ES references (#542)

This is to make those more robust to minor changes on the ES spec.

So for example:

```[=ECMA-262 §9.4.7|ECMA-262 §9.4.7 Immutable prototype exotic objects=]```

Should be something like:

```[=ECMA-262 Immutable prototype exotic objects=]```


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