Remove old documentation from filter stash operations

doc/graph_tool.rst

@@ -149,16 +149,6 @@
        .. automethod:: purge_vertices
        .. automethod:: purge_edges
 
-       .. container:: sec_title
-
-           Stashing and popping the filter state
-
-       .. automethod:: stash_filter
-       .. automethod:: pop_filter
-
-       .. automethod:: get_filter_state
-       .. automethod:: set_filter_state
-
        .. container:: sec_title
 
            I/O operations