feat: Remove unused contacts in housekeeping

[Septias]

Remove contacts that don't have associated chats in housekeeping.

[link2xt]

I commented here #4785 (comment) that it is probably better to remove the contact before deleting files so avatar gets deleted in a single housekeeping run. It is marked as resolved, but seems to be lost during rebase.

[r10s]

Remove contacts that don't have associated chats in housekeeping.

the description is misleading - in that generality, it sounds like a non-goal and i just wanted to comment :)

by reading the code, however, i realised that it is only about contacts that were already manually deleted by the user before :)

as this still sounds fragile, i'd enhance the test to add two contact without a chat before housekeeping:

• add one contact using dc_create_contact() (Claire or so)
• add one contact using dc_add_address_book() (Dave or so)
• check that both contacts exist before housekeeping (creation succeeded)
• check that both contacts still exist after housekeeping (skipped by pruning tombstones)

for blocked contacts, things seem to be fine, they do not use the "origin" (so, blocked contacts do not get the origin "Hidden")

[link2xt]

There is one more problem. What if the contact is removed from all chats, but still has messages where they are author? In this case messages will not be associated to any existing contact anymore.

[Septias]

I guess, the problem is that we have two different requirements for the contact deletion. For the UI, contact deletion should only hide contacts for creating new chats and for the bot (where this issue arose from) deleting contacts should delete everything associated with him.
For the bot code, I now just deleted the 1:1 chat with the user and call delete contact afterward, so all messages & chats should be deleted this way. In my opinion, we could then close this issue because fixing @link2xt problem seems pretty tedious.

[link2xt]

I opened an issue #4792 as this is a problem not only for housekeeping, but for contact deletion as well.

[link2xt]

For the bot code, I now just deleted the 1:1 chat with the user and call delete contact afterward, so all messages & chats should be deleted this way.

There may be messages from this user in other chats, e.g. if someone tries to add the bot to a group.