CTimeProvider: Move global instance to cpp file

This isn't used anywhere externally, so we can keep it hidden in the cpp
file.

Runtime/CTimeProvider.hpp

@@ -1,9 +1,7 @@
 #pragma once
 namespace urde {
-class CTimeProvider;
 class CTimeProvider {
 public:
-  static CTimeProvider* g_currentTimeProvider;
   const float& x0_currentTime; // in seconds
   bool x4_first = true;
   CTimeProvider* x8_lastProvider = nullptr;