Remove implication that nifty is official in the contributions topic

docs/modules/contributions/pages/gui/topic_contributions_gui.adoc

@@ -3,6 +3,5 @@ mitm001
 :description: GUI contributed libraries for the jmonkey engine.
 :keywords: gui, documentation, input, control, hud, contributions
 
-The official engine GUI library is xref:core:gui/nifty_gui.adoc[Nifty GUI]. This
-topic contains user contributed GUI libraries. The most popular and current being
+This topic contains user contributed GUI libraries. The most popular and current being
 Lemur.