Just long press on the clock and drag it to the trash bin. Then get another clock widget that shows only one clock. Don't think about it. You can always get it back on your screen if you want. Its not default. What I mean by default is that, its part of the Touchwiz default set of widgets, widgets that come default with the phone and cannot be uninstalled. I probably should have said "stock", not "default".
My friend's S3 defaulted to another clock widget, not the dual clock, when it was newly opened. I have no idea why yours did. We have 3 Samsung devices at home and non of them had dual clock as the original clock widget when it opened.