Remove the DEFAULT SORTING option from orderby.php

The DEFAULT SORTING option being listed on the shop pages with is a bit redundant - the page loads with the default and the default is listed amongst all the choices.  Having 'DEFAULT SORTING' as it's own option tends to be a bit confusing.  Potentially adding (default) alongside the actual default option's name could be an alternative, but I suspect that once someone has decided to sort a page the way they want it sorted, that they don't often care what the default is.