Remove `beforeAnnotationCreated` event handler in `DocumentMeta`

This handler sets the `document` property of new annotations to metadata
extracted from `<meta>` and `<link>` tags on the page. It is unnecessary
however because the `Guest` class sets this property using either
`DocumentMeta.metadata` or `PDF.getMetadata`, depending on the document
type, before it emits the `beforeAnnotationCreated` event.

In PDFs this code is a serious hazard because if the
`beforeAnnotationCreated` event subscriber in `DocumentMeta` runs before
`AnnotationSync`'s corresponding event handler then DocumentMeta will
overwrite the PDF document-specific annotation metadata with generic
HTML document metadata - losing the critical `documentFingerprint`
property. Fortunately this problem did not occur because the `Guest`
constructor instantiates the `AnnotationSync` class before the
`DocumentMeta` class. Consequently AnnotationSync's
`beforeAnnotationCreated` event handler runs before DocumentMeta's and
pushes new annotations to the sidebar with the correct document metadata
before `DocumentMeta` overwrites it.

src/annotator/plugin/document.js

@@ -72,21 +72,9 @@ export default class DocumentMeta extends Delegator {
     this.document = options.document || document;
     this.normalizeURI = options.normalizeURI || normalizeURI;
 
-    /**
-     * Event handler that adds document metadata to new annotations.
-     */
-    this.beforeAnnotationCreated = annotation => {
-      annotation.document = this.metadata;
-    };
-    this.subscribe('beforeAnnotationCreated', this.beforeAnnotationCreated);
-
     this.getDocumentMetadata();
   }
 
-  destroy() {
-    this.unsubscribe('beforeAnnotationCreated', this.beforeAnnotationCreated);
-  }
-
   /**
    * Returns the primary URI for the document being annotated
    *

src/annotator/plugin/test/document-test.js

@@ -43,12 +43,6 @@ describe('DocumentMeta', function () {
     testDocument.destroy();
   });
 
-  it('attaches document metadata to new annotations', () => {
-    const annotation = {};
-    testDocument.publish('beforeAnnotationCreated', [annotation]);
-    assert.equal(annotation.document, testDocument.metadata);
-  });
-
   describe('annotation should have some metadata', function () {
     let metadata = null;