Fix subwindow settings problem elly found, started in 6cf3437e0.
cmd-pickup: remove cmd_get_arg_item() test in do_cmd_pickup()

This test requires that the command have an item argument, but pickup implicitly
targets the floor and so has no argument.
Merge pull request #311 from elly/pickup-no-arg

cmd-pickup: remove cmd_get_arg_item() test in do_cmd_pickup()