Mark ol.Observable.prototype.unByKey deprecated.

The ol.Observable.prototype.unByKey was supposed to handle removing of
listeners registered using `ol.Observable.prototype.on` and
`ol.Observable.prototype.once`.

However, the underlying google library code called by unByKey only requires
the key, not the `ol.Observable` object on which the registration was
triggered. Moreover, this method was unconvenient and in practice misused.
Example or real world code:
```javascript
var map = new ol.Map();
var key = map.on(listener);
view.unByKey(key);
```

A function was added in #2794, in particular to fix this kind of code.

This PR pushes forward the removal process of the old method in favor of the
unByKey function by marking it `@deprecated` and pointing to the right
usage.

Please merge when deemed appropriate.
The only benefit of `@deprecated` is that it produces a warning if the function is used.