Fix some et-al statements

(I looked at the German author-date styles)

* move the et-al statements from style to citation, bibliography (deutsche-sprache.csl)
* delete et-al-subsequent if it is the same (fachhochschule-vorarlberg.csl, leviathan.csl, sozialwissenschaften-heilmann.csl)
* delete et-al statement in style-node if there are et-al statements in the citation, bibliography-nodes (soziale-welt.csl, sozialpadagogisches-institut-berlin-walter-may.csl)
we should also check this in the journal and/or instructions. 2/1 is _very_ rare as a style requirement, so I'm guessing this is a mistake.