leger lines for 1-line staffs incorrectly offset vertically by 2sp

Just noticed that all those leger lines are offset exactly 2sp.

Marc mentioned in #174966: Single Staff Barlines render incorrect start & stop ypos that there used to be some constant in the code for handling single-line staff. I don't know if that is related, but I'm just crosslinking cause whoever fixes would probably want to tackles both these two issues at the same time.