Filter not wiped from Pogues model

[romaintailhurat]

When an filtered element [0] is deleted via the UI, its filter (or filters) is not deleted from the model.

Step to reproduce:

• create a questionnaire with a sequence S1, and two questions Q1 and Q2
• add a filter on Q2
• save
• delete Q2
• save

Check then that the model still contains the filter (in the form of a FlowControl object in the top-level FlowControl array in the JSON Pogues).