Contact details are still used/visible in app although it's already deleted

[matthesatox]

Describe the bug
Contact details are still used/visible in app although it's already deleted.

To Reproduce
Occurrence: always
Steps to reproduce the behavior:

1. Go to 'contacts' and set for known contact a contact name
2. Start a chat conversation. The contact name is shown in header
3. Go back to contacts and delete the previous entered contact name
4. Go to chat and delete the previous started conversation
5. Go back to contacts and start a new conversation with the same contact.

Actual result
The deleted contact name ist still visible

Expected behavior
The deleted contact name shouldn't be visible

Screenshots
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App information (please complete the following information):

• App version: 0.600.1 (514)
• Plugin version: [e.g. 0.4.0]
• Delta Chat Core version: [e.g. 0.40.0]
• Flutter version: [e.g. 1.0]

Smartphone (please complete the following information):

• Device: iPhone 11 Pro Max
• OS: iOS 13.4 final

Additional context
Add any other context about the problem here.

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