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
Thanks for this feedback.

Honestly, I've been thinking about this comparison and I believe it's better to even drop the last commit, by some reasons

A duck punch on the QUnit logging functions and that would trigger the warning, which would be erroneous and distracting.

Also, it's working this way for ~3 years, it won't log anything if setting a new value instead of registering a new function.