Fix UI freeze when right clicking on a torrent with libtorrent 1.0.x.

Thanks to pmzqla for finding the root issue.
Closes #2365 #2388.

src/core/qtlibtorrent/qbtsession.cpp

@@ -1694,21 +1694,6 @@ void QBtSession::saveFastResumeData() {
     }
 }
 
-bool QBtSession::isFilePreviewPossible(const QString &hash) const {
-    // See if there are supported files in the torrent
-    const QTorrentHandle h = getTorrentHandle(hash);
-    if (!h.is_valid() || !h.has_metadata()) {
-        return false;
-    }
-    const unsigned int nbFiles = h.num_files();
-    for (unsigned int i=0; i<nbFiles; ++i) {
-        const QString extension = fsutils::fileExtension(h.filename_at(i));
-        if (misc::isPreviewable(extension))
-            return true;
-    }
-    return false;
-}
-
 void QBtSession::addTorrentsFromScanFolder(QStringList &pathList)
 {
     foreach (const QString &file, pathList) {

src/core/qtlibtorrent/qbtsession.h

@@ -120,7 +120,6 @@ public:
     ~QBtSession();
     QTorrentHandle getTorrentHandle(const QString &hash) const;
     std::vector<libtorrent::torrent_handle> getTorrents() const;
-    bool isFilePreviewPossible(const QString& hash) const;
     qreal getPayloadDownloadRate() const;
     qreal getPayloadUploadRate() const;
     libtorrent::session_status getSessionStatus() const;

src/gui/transferlistwidget.cpp

@@ -852,7 +852,7 @@ void TransferListWidget::displayListMenu(const QPoint&)
                 has_pause = true;
             }
         }
-        if (h.has_metadata() && BTSession->isFilePreviewPossible(hash) && !has_preview)
+        if (h.has_metadata() && !has_preview)
             has_preview = true;
         first = false;
         if (has_pause && has_start && has_force && has_preview && one_not_seed) break;