Infected AI (Traitor Final Objective) no longer plays the Malf Antag sound effect (#83390)

## About The Pull Request
Does as title says. Infected AI *could* be given a new sound effect but
I'll leave that to someone in the future.

Also removes the ability for Infected AIs to set their own custom
objectives
## Why It's Good For The Game
Getting Malf AI usually means you can ignore laws. AIs really should be
checking their Law 0 before deciding to do their own flavor of chaos but
Infected AI having the same sound effect is an unnecessary extra chance
to mix the two up.
## Changelog
🆑
qol: Traitor-created Infected AIs no longer hear the Malf AI antag sound
alert.
/🆑

---------

Co-authored-by: Afevis <ShizCalev@users.noreply.github.com>

code/modules/antagonists/malf_ai/malf_ai.dm

@@ -20,6 +20,8 @@
 	var/should_give_codewords = TRUE
 	///since the module purchasing is built into the antag info, we need to keep track of its compact mode here
 	var/module_picker_compactmode = FALSE
+	///malf on_gain sound effect. Set here so Infected AI can override
+	var/malf_sound = 'sound/ambience/antag/malf.ogg'
 
 /datum/antagonist/malf_ai/New(give_objectives = TRUE)
 	. = ..()
@@ -39,7 +41,8 @@
 	malfunction_flavor = strings(MALFUNCTION_FLAVOR_FILE, employer)
 
 	add_law_zero()
-	owner.current.playsound_local(get_turf(owner.current), 'sound/ambience/antag/malf.ogg', 100, FALSE, pressure_affected = FALSE, use_reverb = FALSE)
+	if(malf_sound)
+		owner.current.playsound_local(get_turf(owner.current), malf_sound, 100, FALSE, pressure_affected = FALSE, use_reverb = FALSE)
 	owner.current.grant_language(/datum/language/codespeak, source = LANGUAGE_MALF)
 
 	var/datum/atom_hud/data/hackyhud = GLOB.huds[DATA_HUD_MALF_APC]
@@ -272,6 +275,8 @@
 /datum/antagonist/malf_ai/infected
 	name = "Infected AI"
 	employer = "Infected AI"
+	can_assign_self_objectives = FALSE
+	malf_sound = null
 	///The player, to who is this AI slaved
 	var/datum/mind/boss