Attachments: Fix attachments to temporary removed objects

[SmallJoker]

Fixes #8929 by not clearing the attachment upon object removal.

To do

This PR is Ready for Review.

How to test

Imported from #8929:

1. Player 1 sits in a car(t), player 2 watches
2. Player 1 drives away, until either the parent object or the player is unloaded client-side
3. Drive back, and all attachments must be the same

Now do this for various combinations of player_transfer_distance and active_block_range:

• Where former is notable bigger than latter
• Where latter is notable bigger than former

Additional tests: #8701 (to confirm that it still works as intended)

[IhrFussel]

I tested this PR and it fixes my reported bug it seems.

If the cart with the player is too far away the cart unloads and the player model floats above ground so the position even updates when entity is out of sight which is actually something good I think.

[IhrFussel]

This PR makes behavior inconsistent between 5.0.1 and 5.1.0 clients though:

5.1.0 client

• Sees realtime position of attached player model and name tag even after entity unloaded

5.0.1 client

• Only sees last known position of name tag before entity unloaded
• Additionally the name tag moves to ~ 200 nodes away from 0,0,0 after a bit

[sfan5]

We can't fix broken behaviour of older releases of Minetest clients.

[SmallJoker]

@IhrFussel I am well aware of this difference. In fact, I added it on purpose. Do you really want to go back to the buggy behaviour? Doubt it.

EDIT: There are a few more issues regarding unloaded entities but this PR should be enough for the moment.

[sfan5]

Looks good.