yaru: drop self from maintainers

pkgs/data/themes/yaru/default.nix

···
     homepage = "https://github.com/ubuntu/yaru";
     license = with licenses; [ cc-by-sa-40 gpl3Plus lgpl21Only lgpl3Only ];
     platforms = platforms.linux;
-    maintainers = with maintainers; [ moni amaxine ];
+    maintainers = with maintainers; [ moni ];
   };
 }