[WIP][BUGFIX] fixed bug in account contact handling

- fixed bug when removing account contact which is main contact (main contact for the account stayed the same)

__Tasks:__

- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | none
| Fixed tickets | none
| Doc           | none

__Requires:__
- https://github.com/sulu-cmf/SuluContactBundle/pull/138