Move logic for `CBA`s and `MBA`s to target mapping - Remove `Vertragskonto` models

Currently, there are models to model `CBA`s and `MBA`s in `ibims`. This is a feature of the lynqtech system and should be moved to the target models.

Maybe it's senseful to use just the `Contract` models here. You would have to adapt the mapping code in p2l too.

Start by locating the CBA, MBA, and Vertragskonto models in ibims and tracing the mapping code in p2l. Determine how these models are currently represented and which target models should own the logic. Done means the redundant Vertragskonto models and ibims-specific CBA/MBA logic are removed or relocated, with p2l mappings adapted accordingly.

Written by the indexing model from the issue text.