Fix bug for the remove method of BinarySearchTree

[zhijli]

Fix two bugs for the remove method:

Bug 1: When the node to be removed has two children , should remove the successor (nextLarger) node, not the right child.
For example, given a tree (A), if we remove node 2, the tree should looks like (B), but now it actually looks like (C).

         (A)                   (B)                    (C)
          8                     8                      8
         / \                   / \                    / \ 
        2   9                 3   9                  5   9        
       / \                   / \                    / \
      1   5                 1   5                  1   6
         / \                     \                    /   
        3   6                     6                  3      

Bug 2: If we remove the root of the tree, should assign the new child node to the root.
For exmaple, given a tree (A), if we remove node 6, it should looks like (B), but now it looks like (C).

      (A)                   (B)                (C)
Root ->  6             Root -> 8         Root -> 6
          \                   / \
           8                 7   9                8
          / \                                    / \
         7   9                                  7   9

[aalhour]

Nice catch, thanks for contributing!