[coco] Invoke detach from each Child class (#1713)

[coco] Invoke detach from each Child class (#1713)

* [coco] Invoke detach from each Child class

The current implementation invokes 'detach' inside the destructor of Node.

Unfortunately, this may result in undefined behavior as detach may access the
internal of Child class (through leaving hook), but Child class is already
destructed.

With this commit, unlink on destruction is the responsibility of each
Child class.

Signed-off-by: Jonghyun Park <jh1302.park@samsung.com>
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