fix compatibility for RemoteDirNode

Author: rchevrier

src/fcollections/core/_listing.py

@@ -264,7 +264,6 @@ def children(self) -> tp.Iterator[INode]:
             The child nodes, either files or folders
         """
 
-    @abc.abstractmethod
     def clear(self):
         """Clear child nodes."""
 
@@ -285,9 +284,6 @@ def children(self) -> tp.Iterator[INode]:
         """
         return []
 
-    def clear(self):
-        pass
-
 
 class DirNode(INode):
     """Directory node of a file system tree.