Remove duplicate css class.

packages/default-theme/style/icons.css

@@ -27,12 +27,6 @@
   background-image: url(icons/md/close.svg);
 }
 
-
-.jp-CloseIcon {
-  background-image: url(icons/md/close.svg);
-}
-
-
 .jp-CutIcon {
   background-image: url(icons/md/cut.svg);
 }
@@ -159,4 +153,4 @@
 
 .jp-FilledCircleIcon {
   background-image: url(icons/md/lens.svg);
-}
+}