Remove dup code

Author: phofl

pandas/core/groupby/groupby.py

@@ -2733,9 +2733,6 @@ def nth(
 
                 ids, _, _ = self.grouper.group_info
 
-                # Drop NA values in grouping
-                mask = mask & (ids != -1)
-
                 out = self._mask_selected_obj(mask)
                 if not self.as_index:
                     return out