Remove constructor

The removed `F()` constructor were implemented on @1039a2c, to tell if the
callback methods have been overwritten on the actual QUnit object.

It doesn't work for backwards compatibility and the old and deprecated
way to assign callbacks doesn't work anymore.

Making this change won't affect anything on the current API, even
assigning a new value on any callback function. The effects are the
same with or without this commit, proving the removed code is useless.

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Also two additional code cleanup commits
What's the goal?

If it is to make consumers using the old callback style upgrade to the modern style, we should just stop them dead in their tracks by throwing an Error.

If it is to fix the backward compatibility that we promised, then we should do so by re-registering it for them AND issuing a non-fatal `console.warn`/`console.error` message.

To a consumer using the old style, this new behavior is arguably even more confusing than what is happening right now.