Remove Promise.prototype.finally calls from translate.js not in ES6

We could monkey-patch a finally function in connectors as described in
this gist https://gist.github.com/jish/e9bcd75e391a2b21206b , but it
appears to silence propogation of unhandled rejections
and it could later come back to bite us in the butt.
Native and proper support for finally is, however, planned in ES7.

chrome/content/zotero/xpcom/translation/translate.js

@@ -1542,7 +1542,7 @@ Zotero.Translate.Base.prototype = {
 					this._waitingForSave = true;
 					this._saveItems(this.saveQueue)
 						.catch(e => this._runHandler("error", e))
-						.finally(() => this.saveQueue = []);
+						.then(() => this.saveQueue = []);
 					return;
 				}
 				this._debug("Translation successful");