Hello,

In my opinion, the simpliest way is to select your initial object (right shoe), save it as obj, clear the scene, load again it and do a mirror copy.

If it's not possible, another way: You should have now one object (mirrored shoes) in your scene. In the scene tab, just lock and hide your object. All the unwanted polys should now appears as standalone polys (as they were from a different copy).

Go in poly mode, select all and delete. Unhide and  unlock your object, it should be OK.

Last solution: Select by material and delete manualy your polys (but i prefer the first solution...i'm a lazy boy :-))

Hope it helps,