Remove DiffCacheContent.isNoDifference

We stopped using this method in 2fd32f8616
when Cedric removed the NoDifferenceException and its related dialog.

Signed-off-by: Shawn O. Pearce <sop@google.com>

src/main/java/com/google/gerrit/server/patch/DiffCacheContent.java

@@ -119,10 +119,6 @@ public final class DiffCacheContent implements Serializable {
     }
   }
 
-  public boolean isNoDifference() {
-    return header == null;
-  }
-
   public ObjectId getOldId() {
     return oldId;
   }