base_sink default ctor

include/spdlog/sinks/base_sink.h

@@ -22,11 +22,7 @@ template<typename Mutex>
 class base_sink : public sink
 {
 public:
-    base_sink()
+    base_sink() = default;
-        : sink()
-    {
-    }
-
     base_sink(const base_sink &) = delete;
     base_sink &operator=(const base_sink &) = delete;