Remove unused "remove reviewer" image

The image was replaced by a unicode character in Ib314d116f6 and is
now unused. Remove it.

Change-Id: Idcc445b73d4a2d82729ea19f09f058b3a613bff3

gerrit-gwtui-common/src/main/java/com/google/gerrit/client/Resources.java

@@ -81,9 +81,6 @@ public interface Resources extends ClientBundle {
   @Source("merge.png")
   public ImageResource merge();
 
-  @Source("removeReviewer.png")
-  public ImageResource removeReviewer();
-
   @Source("deleteNormal.png")
   public ImageResource deleteNormal();