Revert "added linkedlist abstract method (codezonediitj#223)"

This reverts commit 6a9f06e.

pydatastructs/linear_data_structures/linked_lists.py

@@ -43,66 +43,6 @@ def __str__(self):
                 break
         return str(elements)
 
-    def insert_after(self, prev_node, key, data=None):
-        """
-        Inserts a new node after the prev_node.
-
-        Parameters
-        ==========
-
-        prev_node: LinkedListNode
-            The node after which the
-            new node is to be inserted.
-
-        data
-            Any valid data to be stored in the node.
-        """
-        raise NotImplementedError('This is an abstract method')
-
-    def insert_at(self, index, key, data=None):
-        """
-        Inserts a new node at the input index.
-
-        Parameters
-        ==========
-
-        index: int
-            An integer satisfying python indexing properties.
-
-        data
-            Any valid data to be stored in the node.
-        """
-        raise NotImplementedError('This is an abstract method')
-
-    def extract(self, index):
-        """
-        Extracts the node at the index of the list.
-
-        Parameters
-        ==========
-
-        index: int
-            An integer satisfying python indexing properties.
-
-        Returns
-        =======
-
-        current_node: LinkedListNode
-            The node at index i.
-        """
-        raise NotImplementedError('This is an abstract method')
-
-    def __getitem__(self, index):
-        """
-        Returns
-        =======
-
-        current_node: LinkedListNode
-            The node at given index.
-        """
-        raise NotImplementedError('This is an abstract method')
-
-
 class DoublyLinkedList(LinkedList):
     """
     Represents Doubly Linked List