[webcomponents] [Custom] attachedCallback and detechedCallback should be removed (#286)

In addition #222, `attachedCallback` and `detachedCallback` should be removed.

While it's a viable optimization to avoid work in a custom element while the element is not in the document, there is a lot of work that needs to happen regardless of where an element is inserted.

Since the author can easily check the existence of the browsing context by checking `document.defaultView` or detect whether the element has `display: none` as the used value or not.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/286