Remove sequential operator and meta-state

In the latest formalization of VOs, sequential composition is done by
passing a validation task/expression as an argument to another task.
This makes the previous "animator meta-state" concept obsolete.

Because this parser doesn't handle validation task arguments yet (and we
still have work to do before we can implement that usefully), there is
nothing to be type-checked at this stage anymore, so I'm removing the
type checker completely for now.

The first implementation of the new type checker will probably be inside
ProB 2 UI. Once we have formalized the validation task types and their
parameters well enough, we can implement a standalone type checker in
this library again.