Remove unused declaration

src/converse-chatview.js

@@ -498,7 +498,6 @@
                      *      attributes.
                      */
                     const current_msg_date = moment(attrs.time) || moment,
-                          prepend_html = _.bind(this.content.insertAdjacentHTML, this.content, 'afterbegin'),
                         previous_msg_date = this.getLastMessageDate(current_msg_date),
                         message_el = this.renderMessage(attrs);