Remove unused file with obsolete PenWindows support.

This file was apparently never compiled in, just drop it, as well as the
mention of wxUSE_PENWINDOWS in the documentation.

docs/doxygen/mainpages/const_wxusedef.h

@@ -334,7 +334,6 @@ manifest from wxWidgets RC file. See also wxUSE_RC_MANIFEST.}
 @itemdef{wxUSE_OLE, Enables OLE helper routines.}
 @itemdef{wxUSE_OLE_AUTOMATION, Enable OLE automation utilities.}
 @itemdef{wxUSE_OLE_CLIPBOARD, Use OLE clipboard.}
-@itemdef{wxUSE_PENWINDOWS, See src/msw/penwin.cpp file.}
 @itemdef{wxUSE_POSTSCRIPT_ARCHITECTURE_IN_MSW, Use PS printing in wxMSW.}
 @itemdef{wxUSE_PS_PRINTING, See src/msw/dcprint.cpp file.}
 @itemdef{wxUSE_RC_MANIFEST, Include manifest for common controls library v6