Remove redundant tests for __WIN32__ in wxMSW code.

This is always defined, Win16 is not supported since ages and Win64 defines
both __WIN32__ and __WIN64__.

src/msw/metafile.cpp

@@ -326,7 +326,6 @@ void wxMetafileDCImpl::SetMapMode(wxMappingMode mode)
 // wxMakeMetafilePlaceable
 // ----------------------------------------------------------------------------
 
-#ifdef __WIN32__
 struct RECT32
 {
   short left;
@@ -343,16 +342,6 @@ struct mfPLACEABLEHEADER {
     DWORD    reserved;
     WORD    checksum;
 };
-#else
-struct mfPLACEABLEHEADER {
-    DWORD    key;
-    HANDLE    hmf;
-    RECT    bbox;
-    WORD    inch;
-    DWORD    reserved;
-    WORD    checksum;
-};
-#endif
 
 /*
  * Pass filename of existing non-placeable metafile, and bounding box.