Remove outdated comment
Remove outdated comment

With the newest rewrite of `MultiColumnJoin::getVariableColumns()` the removed comment became outdated. I think the code is simple enough to go without a comment. This is especially true as the removed comment was hard to read anyway.
Buchhold/QLever,Buchhold/QLever,Buchhold/QLever,Buchhold/QLever,Buchhold/QLever