drop the string initialization of Tree

nltk/test/tree.doctest

@@ -39,39 +39,6 @@ tree object to one of several standard tree encodings:
             [.dp [.d the ] [.np dog ] ]
             [.vp [.v chased ] [.dp [.d the ] [.np cat ] ] ] ]
 
-Trees can be initialized from treebank strings:
-
-    >>> tree2 = Tree('(S (NP I) (VP (V enjoyed) (NP my cookie)))')
-    >>> print(tree2)
-    (S (NP I) (VP (V enjoyed) (NP my cookie)))
-
-Trees can be compared for equality:
-
-    >>> tree == Tree(str(tree))
-    True
-    >>> tree2 == Tree(str(tree2))
-    True
-    >>> tree == tree2
-    False
-    >>> tree == Tree(str(tree2))
-    False
-    >>> tree2 == Tree(str(tree))
-    False
-
-    >>> tree != Tree(str(tree))
-    False
-    >>> tree2 != Tree(str(tree2))
-    False
-    >>> tree != tree2
-    True
-    >>> tree != Tree(str(tree2))
-    True
-    >>> tree2 != Tree(str(tree))
-    True
-
-    >>> tree < tree2 or tree > tree2
-    True
-
 Tree Parsing
 ============
 
@@ -1075,4 +1042,3 @@ This used to discard the ``(B b)`` subtree (fixed in svn 6270):
     >>> print(Tree.fromstring('((A a) (B b))'))
     ( (A a) (B b))
 
-