The new completion API no longer needs either of these. Instead of
referencing an enum member, cmdutils.argument.completion now points to
a function that returnsthe desired completion model.
This vastly simplifies the addition of new completion types. Previously
it was necessary to define the new model as well as editing usertypes
and completion.models.instances. Now it is only necessary to define a
single function under completion.models.
This is the next step of Completion Model/View Revamping (#74).
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