Remove support for wxMotif

This port hasn't been updated for ages and is not used by anybody any
longer, so remove its code to facilitate maintenance.

Also remove references to this port from the documentation and most of
the other places (VMS-specific descrip.vms files still check for it
because it's not clear how to update them all), including configure.

Regenerate the latter and rebake all makefiles.

Finally document that this port is not available any longer.

samples/dragimag/dragimag.cpp

@@ -400,7 +400,7 @@ bool MyApp::OnInit()
     }
 
 #if 0
-    // Under Motif or GTK, this demonstrates that
+    // Under GTK, this demonstrates that
     // wxScreenDC only gets the root window content.
     // We need to be able to copy the overall content
     // for full-screen dragging to work.

samples/dragimag/makefile.unx

@@ -25,7 +25,7 @@ LDFLAGS ?=
 # Location and arguments of wx-config script 
 WX_CONFIG ?= wx-config
 
-# Port of the wx library to build against [gtk1,gtk2,msw,x11,motif,osx_cocoa,osx_carbon,dfb]
+# Port of the wx library to build against [gtk1,gtk2,msw,x11,osx_cocoa,osx_carbon,dfb]
 WX_PORT ?= $(shell $(WX_CONFIG) --query-toolkit)
 
 # Use DLL build of wx library to use? [0,1]