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
> A duck punch on the QUnit logging functions and that would trigger the warning, which would be erroneous and distracting.

Great point (though I suspect it would be pretty rare to find someone duck-punching those particular methods).

> 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.

A fair point as well, though when it comes to open source libraries, I personally believe that version numbers are more important than time. For example, it is [unfortunately] not uncommon for a project to use the latest version of a library at the time it is under active development, then go into maintenance/hold mode, and then come back under active development with new developers. When the new devs [hopefully] go to upgrade the libraries, the version number will have barely changed (and still be within the same major version) but the functionality the old devs used is now broken.