Fix duplicate registrations removal not being commited (#777)

Author: gdaviet

collectives/models/event/registration.py

@@ -346,8 +346,10 @@ def add_registration_check_race_conditions(
         if registration.is_duplicate():
             self.registrations.remove(registration)
             db.session.delete(registration)
+            db.session.commit()
             raise DuplicateRegistrationError("Vous êtes déjà inscrit(e).")
         if not allow_overbooking and registration.is_overbooked():
             self.registrations.remove(registration)
             db.session.delete(registration)
+            db.session.commit()
             raise OverbookedRegistrationError("L'événement est déjà complet.")