[ImgBot] Optimize images

*Total -- 20.10kb -> 7.66kb (61.9%)

/lcof/面试题68 - II. 二叉树的最近公共祖先/images/binarytree.png -- 7.35kb -> 2.62kb (64.37%)
/lcof/面试题68 - I. 二叉搜索树的最近公共祖先/images/binarysearchtree_improved.png -- 7.48kb -> 2.68kb (64.13%)
/solution/0200-0299/0237.Delete Node in a Linked List/images/237_example.png -- 5.26kb -> 2.35kb (55.31%)

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