Allow removing of schema constraints

PR #923 introduced schema constraints for vertex and edge properties, as well as for allowed connections of edges. Unfortunately, it's currently not possible with the `ManagementSystem` to remove these constraints when the schema changed.
This means that schema evolvement currently only works in the direction of allowing new constraints but existing ones cannot be removed when they don't match the new schema anymore.

Start by reading PR #923 and the ManagementSystem referenced in the issue to understand how vertex, edge, and allowed-connection constraints are represented. Done means schema evolution can remove constraints that no longer match the new schema, not only add newly allowed constraints.

Written by the indexing model from the issue text.