Remove VtUnknown classification

This clean-up removes the dependency of the current proof mode (and hence
the parsing state) on unification.

The current proof mode can now be known simply by parsing and elaborating
attributes. We give access to attributes from the classifier for this purpose.

We remove the infamous `VtUnknown` code path in the STM which is known to
be buggy.

Fixes #3632 #3890 #4638.

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