Remove group members only from specific group instead of all groups

Bugfix for !1199 (merged)

What does this MR do?

See title.

How confident are you it won't break things if deployed?

Very

Links to related issues

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally (or use beta)
2. Login as workgroup admin
3. Remove a member
4. See if this member still has all districts in profile

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings