Delete exported custom anchors

[jyasskin]

The <pre class="anchors"> section is huge and includes lots of terms whose <dfn> includes data-export. (e.g. https://html.spec.whatwg.org/multipage/webappapis.html#relevant-settings-object, https://html.spec.whatwg.org/multipage/nav-history-apis.html#window-bc, https://url.spec.whatwg.org/#concept-url-origin) You should delete those because having a custom anchor for a term prevents the term from following moves in the source document.

For non-exported terms in web specs, it's best to file an issue or PR asking to export the term. Sometimes the editor will point out a better way to integrate with their spec.