[clipboard-apis] Kill onbeforecopy, onbeforecut and onbeforepaste events (#25)

These events don't do a good job at fulfilling their use cases, have pretty much no good implementation and are confusingly named (making people think they resemble an event like beforeInput).

We can watch https://github.com/w3c/editing/issues/93 to see if the editing group comes up with a generic "let the browser know that command x is enabled/disabled" feature we can use instead.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/clipboard-apis/issues/25