Core/Utilities: Remove global using of std::chrono namespace.

* never use "using namespace" in header files!

(cherry picked from commit 6bf10c2b83063147737aee68d3b40a6d67e49d53)

src/server/game/Server/WorldSocket.cpp

@@ -509,6 +509,8 @@ void WorldSocket::SendAuthResponseError(uint8 code)
 
 bool WorldSocket::HandlePing(WorldPacket& recvPacket)
 {
+    using namespace std::chrono;
+
     uint32 ping;
     uint32 latency;
 

src/server/shared/Utilities/Timer.h

@@ -21,10 +21,10 @@
 
 #include <chrono>
 
-using namespace std::chrono;
-
 inline uint32 getMSTime()
 {
+    using namespace std::chrono;
+
     static const system_clock::time_point ApplicationStartTime = system_clock::now();
 
     return uint32(duration_cast<milliseconds>(system_clock::now() - ApplicationStartTime).count());