Merge branch 'thr_bitree' of https://github.com/hekun/dsc into thr_bitree
删除Threaded_bitree.c文件。因为线索二叉树操作函数的实现均位于thread_bitree.c文件。
Merge branch 'thr_bitree' of https://github.com/hekun/dsc into thr_bitree