[no GBP] Corrects the speed at which spiders stop being on fire (#73601)

## About The Pull Request

As noted in #73584 spiders stopped being on fire faster when they were
simple mobs than they do now, and I have restored this behaviour.
While I was there I noticed that spiders were the only simple mobs which
could catch fire so I just remove that behaviour.
I also changed the boolean "flammable" into a flag, not sure if that's
actually better but maybe it is? I tried a little to see if I could make
this into a component but there's basically no way as all of this
behaviour is inside a status effect.

## Why It's Good For The Game

Restores original behaviour.
Maybe in the future I'll (or someone else will) use this to make some
other basic mobs able to be set on fire, no obvious reason they
shouldn't be able to be.

## Changelog

🆑
fix: Spiders will stop being on fire marginally faster, as they used to.
/🆑

code/modules/mob/living/basic/basic.dm

@@ -7,6 +7,7 @@
 	gender = PLURAL
 	living_flags = MOVES_ON_ITS_OWN
 	status_flags = CANPUSH
+	fire_stack_decay_rate = -5 // Reasonably fast as NPCs will not usually actively extinguish themselves
 
 	var/basic_mob_flags = NONE
 
@@ -97,9 +98,6 @@
 	///This damage is taken when the body temp is too hot. Set both this and unsuitable_cold_damage to 0 to avoid adding the basic_body_temp_sensitive element.
 	var/unsuitable_heat_damage = 1
 
-	///Whether or not this mob can catch on fire
-	var/flammable = FALSE
-
 /mob/living/basic/Initialize(mapload)
 	. = ..()