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/common/logging/log.h

@@ -78,11 +78,7 @@ enum class Class : ClassType {
     Count ///< Total number of logging classes
 };
 
-/**
- * Logs a message to the global logger. This proxy exists to avoid exposing the details of the
- * Logger class, including the ConcurrentRingBuffer template, to all files that desire to log
- * messages, reducing unecessary recompilations.
- */
+/// Logs a message to the global logger.
 void LogMessage(Class log_class, Level log_level,
     const char* filename, unsigned int line_nr, const char* function,
 #ifdef _MSC_VER