Core: Remove the newly introduced global hooks until we have consensus

Between locating the hooks in `QUnit` or `QUnit.config` and making them simple setters and callback lists (like QUnit.done et al) and upcoming plans for nested suites, we decided not to release this feature, for now.

I'm keeping the abstractions for hooks in place, so it should be trivial to bring this back in whatever form we decide on later.

Effectively reverts 5ee31a083af3b17187f90cde54885bcc46d3dd8f and follow-up commits.

Fixes #665
Ref #633
Ref #635
Ref #647
these context tests are important.