One way would be to do:
setof(elem lst (count++!=n))
The second argument is the position to remove, where 0 is the first entry in the list.
Another alternative is to use destructive methods:
I've not profiled these, but the second generates less garbage, because it modifies the list in place, whereas the first constructs a new list.
That said, it's a slightly odd thing to do - generally if you're accessing lists via a numbered index, it often means you're using the wrong approach. Lists are sequential data structures, and so are not really well suited to be accessed in a "random access" manner.