removed lock from dist_sink::_flush() (moved to base_sink::flush())

include/spdlog/sinks/dist_sink.h

@@ -46,7 +46,6 @@ protected:
 
     void _flush() override
     {
-        std::lock_guard<Mutex> lock(base_sink<Mutex>::_mutex);
         for (auto &sink : _sinks)
             sink->flush();
     }