Deprecate PQ22DLocalFiniteElement

Carsten Gräserrequested to merge
deprecate/pq22d into master

This class has originally been introduced to demonstrate how to manually implement a local finite element for a mixed grid. Today you can implement mixed elements easily using LocalFiniteElementVariant or LocalFiniteElementVirtualInterface. Since we don't have such classes for other cases (1d, 3d, order!=2) I don't see why we need this one.

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