Drop access to native window.

The new NFD library is not using this information, and the old one was using
it only on Windows. Oh well.

Removal of this functionality also removes some build-time decisions.

server/TracySourceView.hpp

@@ -141,9 +141,7 @@ private:
     };
 
 public:
-    using GetWindowCallback = void*(*)();
-
-    SourceView( GetWindowCallback gwcb );
+    SourceView();
 
     void UpdateFont( ImFont* fixed, ImFont* small ) { m_font = fixed; m_smallFont = small; }
     void SetCpuId( uint32_t cpuid );
@@ -259,7 +257,6 @@ private:
     float m_asmWidth;
     float m_jumpOffset;
 
-    GetWindowCallback m_gwcb;
     Tokenizer m_tokenizer;
 
     struct