Some death messages are printed twice

### Describe the bug

When you kill a panicked person, or a petrified person, their death message is printed twice
This _may_ be related to guilt assignment, or at the very least the `death_function` field (see Additional context)

### Attach save file

N/A

### Steps to reproduce

1. Create a new world, character, etc
2. Debug a `mon_civilian_panic` aka "panicked person" or a `mon_civilian_stationary` aka "petrified person" and kill it, either directly or by debug -> Map... -> Kill all monsters
3. The death message is printed twice, with the guilt message printed in between
4. Try 1 - 3 having Killer Drive or Uncaring. The guilt message is ommited, so the death message is printed as "The petrified person dies! x 2"
5. Try 1 - 3 with a `mon_mutant_alpha_boss` aka "mutant child", see how the `death_function` message is printed first, and the default monster death message is printed after

### Expected behavior

The death message should only be printed once

### Screenshots

After killing a single panicked person

After killing a single petrified person with the Debug method

After adding the Uncaring trait by Debug, and killing a single petrified person

After trying the same with `mon_mutant_alpha_boss`

### Versions and configuration

- OS: Windows
- OS Version: 10.0.19044.2965 (21H2)
- Game Version: 9437078 [64-bit]
- Graphics Version: Tiles
- Game Language: English [en]
- Mods loaded: [
Dark Days Ahead [dda],
Disable NPC Needs [no_npc_food],
Portal Storms Ignore NPCs [personal_portal_storms],
Slowdown Fungal Growth [no_fungal_growth]
]

### Additional context

This was noticed in #71619 by two people but it wasn't reported as a bug.

I'm guessing it's caused by the presence of the `death_function` field, which is _adding_ a death message, instead of replacing it. Having a `death_function` with no message prints _a_ default monster death message (The %s dies!), and then displays _the_ default monster death message -> the `P` window merges both and displays them as `The %s dies! x 2`. This is the case for the panicked and petrified persons.
However, having a `death_function: message`, such as with the mutant child, causes the custom message to be printed first, then the default monster death message is displayed.

Start by tracing death handling for the mon_civilian_panic and mon_civilian_stationary cases, focusing on how death_function and the default monster death message are processed. Reproduce the issue by killing these monsters through the Debug Map kill-all action, then compare with mon_mutant_alpha_boss. Done means each death displays exactly one message, while a custom death_function message remains correct.

Written by the indexing model from the issue text.