Get rid of a use of WXSTRINGCAST from src/msw/metafile.cpp

WXSTRINGCAST is a const-removing cast and should be avoided unless necessary.

Here, wxString::c_str() is sufficient because GetTextExtentPoint32 takes
a const pointer.

src/msw/metafile.cpp

@@ -236,7 +236,7 @@ void wxMetafileDCImpl::DoGetTextExtent(const wxString& string,
 
     SIZE sizeRect;
     TEXTMETRIC tm;
-    ::GetTextExtentPoint32(dc, WXSTRINGCAST string, wxStrlen(WXSTRINGCAST string), &sizeRect);
+    ::GetTextExtentPoint32(dc, string.c_str(), string.length(), &sizeRect);
     ::GetTextMetrics(dc, &tm);
 
     if ( x )