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
That's the argument I've been missing. 

I'll update my last commit to re-register the callback based on your suggestion. We can drop it on 2.0.0