B2OC: Fix make_b2cstarhh to apply vertex separation cut on B and D0 from Dstar (shifters please ignore)

In !2205 (merged), some temporary cuts were applied in make_b2cstarhh to reduce the rates of some lines following the improvements to the Dstar selections, and corresponding efficiency increases, also made in that MR. One of these cuts was on the vertex separation in z between the b-hadron and excited c-hadron, which was previously disabled. This MR fixes this by instead applying the cut on the separation between the b-hadron and ground-state c-hadron that comes from the excited c-hadron decay.

cc: @mwhitehe