Searchlights: don't drop items used for settings

[ehughsbaird]

Summary

None

Purpose of change

Fixes #63333

Describe the solution

Searchlights store their settings on a processor board item, so their "beams" can have persistent behavior between turns. They do this using item vars, but these item vars linger after their death, causing the items not to stack.

Simple solution: Just don't drop these settings items. Also, erase these item vars on any existing items, so that they can now stack.

Describe alternatives you've considered

Removing these vars on death would be an alternate option, but placing it with where the item vars are set shows future contributors who might use this code as inspiration that this is a problem. It also avoids being a problem when (if) the searchlights are made to drop a disassembleable robot corpse in the future like other robots do.

Testing

No settings item fix:

1. Spawn in several searchlights.
2. Wait five minutes.
3. Kill the searchlights.
4. Searchlights will only drop processor boards as random draws from their death drops. These will stack with other processor boards.

Stacking fix:

1. Acquire a savegame with non-stacking processor boards from searchlights.
2. Load that savegame, and those processor boards will now stack.

Additional Context

This does mean that searchlights are no longer guaranteed to drop multiple processor items. I think that this is acceptable.