Use non-recursive mutex

The related classes doesn't really need recursive mutex so drop it. And
if it is really required it usually indicates bad design and we should
avoid it anyway.

src/base/settingsstorage.cpp

@@ -155,7 +155,6 @@ SettingsStorage *SettingsStorage::m_instance = nullptr;
 SettingsStorage::SettingsStorage()
     : m_data{TransactionalSettings(QLatin1String("qBittorrent")).read()}
     , m_dirty(false)
-    , m_lock(QReadWriteLock::Recursive)
 {
     m_timer.setSingleShot(true);
     m_timer.setInterval(5 * 1000);