ENH: Remove convenience functions

[MImmesberger]

In the Sozialversicherungsbeiträge directory, there are convenience functions (betrag_gesamt_m in particular). They're there for convenience, serving as a target to get the sum of employer and employee contributions to social insurance. Because they are not directly related to a tax or transfer, they should be removed.

[hmgaudecker]

Let's leave them in. Now that they'll be called beiträge after #841, it'll be clear enough.