Don't "export" X11 specific fl_open_display() function

The X11 specific overloaded function fl_open_display(Display *)
has never been documented but exposed (and "exported" in the ABI)
in FL/platform.H which is not necessary. This internal function
is now static and no longer exposed in the ABI.

Don't confuse with fl_open_display() w/o arguments which is still
exported and declared in FL/platform.H.

FL/platform.H

@@ -3,17 +3,17 @@
 //
 // Platform header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2018 by Bill Spitzak and others.
+// Copyright 1998-2020 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
 // file is missing or damaged, see the license at:
 //
-//     http://www.fltk.org/COPYING.php
+//     https://www.fltk.org/COPYING.php
 //
 // Please report all bugs and problems on the following page:
 //
-//     http://www.fltk.org/str.php
+//     https://www.fltk.org/str.php
 //
 
 // These are FLTK symbols that are necessary or useful for calling
@@ -57,8 +57,6 @@ class Fl_Window;
 #    endif
 #    include <X11/Xatom.h>
 
-FL_EXPORT void fl_open_display(Display*);
-
 // constant info about the X server connection:
 extern FL_EXPORT Display *fl_display;
 extern FL_EXPORT int fl_screen;