MlkBookmarkSubmissionForm: Implement disconnectedCallback().

Change-Id: Id93112b47d092711cd533c678901177fa73e3255
diff --git a/src/main/resources/META-INF/resources/bookmarks/MlkBookmarkSubmissionForm.js b/src/main/resources/META-INF/resources/bookmarks/MlkBookmarkSubmissionForm.js
index 4d9b40c..520aa32 100644
--- a/src/main/resources/META-INF/resources/bookmarks/MlkBookmarkSubmissionForm.js
+++ b/src/main/resources/META-INF/resources/bookmarks/MlkBookmarkSubmissionForm.js
@@ -70,6 +70,10 @@
     this.descriptionInput.innerText = this.description || "";
   }
 
+  disconnectedCallback () {
+    this.uriInput.removeEventListener('blur', this.onUriBlur.bind(this));
+  }
+
   attributeChangedCallback(name /*:string*/, oldValue /*:string*/, newValue /*:string*/) {
   }