[Trunk] Delete tie or slur in linked parts doesn't work

Trunk r4801

1. Create a score with 1 linked staff, add a couple of notes and tie or slur or open the attached file
2. Select the tie and press delete

Expected result : the tie is removed from both staves.
Actual result : the tie is only removed from one staff.
(Same with slur)

Side note : it's not easy to delete the tie, if you click on it and press delete, it doesn't always work.