Remove casting from previous commit and fix the following Klockwork issues:

1. Removing "return" from void functions.
2. Using "const" for operator= argument.

include/spdlog/sinks/sink.h

@@ -15,7 +15,10 @@ namespace sinks
 class sink
 {
 public:
-    sink() { _level = (int)level::trace; }
+    sink()
+    {
+         _level = level::trace;
+    }
 
     virtual ~sink() {}
     virtual void log(const details::log_msg& msg) = 0;