Don't leak a hidden browser for every single imported text/HTML file

It is possible we have been doing this for a very long time.

(Only local imports were affected, not snapshot saving.)

chrome/content/zotero/xpcom/attachments.js

@@ -1345,6 +1345,7 @@ Zotero.Attachments = new function(){
 				// Chain fulltext indexer inside the charset callback,
 				// since it's asynchronous and a prerequisite
 				Zotero.Fulltext.indexDocument(browser.contentDocument, itemID);
+				Zotero.Browser.deleteHiddenBrowser(browser);
 			}
 			
 			// Since the callback can be called during an import process that uses