Stop passing WNDPROC argument to wxCheckWindowWndProc()

This parameter wasn't used anyhow and casting wxWndProc or m_oldWndProc
to WXFARPROC just resulted in gcc8 -Wcast-function-type warnings.

Get rid of them by not passing the window proc to this function at all.

include/wx/msw/private.h

@@ -936,7 +936,10 @@ extern WXDLLIMPEXP_CORE int wxGetWindowId(WXHWND hWnd);
 
 // check if hWnd's WNDPROC is wndProc. Return true if yes, false if they are
 // different
-extern WXDLLIMPEXP_CORE bool wxCheckWindowWndProc(WXHWND hWnd, WXFARPROC wndProc);
+//
+// wndProc parameter is unused and only kept for compatibility
+extern WXDLLIMPEXP_CORE
+bool wxCheckWindowWndProc(WXHWND hWnd, WXFARPROC wndProc = NULL);
 
 // Does this window style specify any border?
 inline bool wxStyleHasBorder(long style)

src/msw/window.cpp

@@ -1252,7 +1252,7 @@ void wxWindowMSW::SubclassWin(WXHWND hWnd)
 
     // we don't need to subclass the window of our own class (in the Windows
     // sense of the word)
-    if ( !wxCheckWindowWndProc(hWnd, (WXFARPROC)wxWndProc) )
+    if ( !wxCheckWindowWndProc(hWnd) )
     {
         wxSetWindowProc(hwnd, wxWndProc);
 
@@ -1289,7 +1289,7 @@ void wxWindowMSW::UnsubclassWin()
 
         if ( m_oldWndProc )
         {
-            if ( !wxCheckWindowWndProc((WXHWND)hwnd, m_oldWndProc) )
+            if ( !wxCheckWindowWndProc((WXHWND)hwnd) )
             {
                 wxSetWindowProc(hwnd, (WNDPROC)m_oldWndProc);
             }