don't leave unencrypted chats

in unnencrypted chats, core shouldn't send "foo left the group" messages, so trying to leave them should fail

this issue was created from @Hocuri's comment at https://github.com/deltachat/deltachat-android/issues/4158#issuecomment-3769663920

> I didn't even know that we allow "leaving" unencrypted chats. I think it would even be fine to disallow "leaving" an unencrypted chat - you can't add members there, and we don't want users to accidentally confirm to a spammer that this email address is in use. Or keep things as they are for unencrypted chats, it's not too important what we do for them.