Common: Remove async logging

It provided a large increase in complexity of the logging system while
having a negligible performance impact: the usage patterns of the ring
buffer meant that each log contended with the logging thread, causing
it to effectively act as a synchronous extra buffering.

Also removed some broken code related to filtering of subclasses which
was broken since it was introduced. (Which means no one ever used that
feature anyway, since, 8 months later, no one ever complained.)

src/citra_qt/main.cpp

@@ -350,14 +350,8 @@ void GMainWindow::closeEvent(QCloseEvent* event)
 
 int main(int argc, char* argv[])
 {
-    std::shared_ptr<Log::Logger> logger = Log::InitGlobalLogger();
     Log::Filter log_filter(Log::Level::Info);
     Log::SetFilter(&log_filter);
-    std::thread logging_thread(Log::TextLoggingLoop, logger);
-    SCOPE_EXIT({
-        logger->Close();
-        logging_thread.join();
-    });
 
     QApplication::setAttribute(Qt::AA_X11InitThreads);
     QApplication app(argc, argv);