Fixes sentries firing turning off their lights (#4010)

# About the pull request

The muzzle flash is already lower than the inbuilt illum so it's just
turning off the intended light when firing.

# Explain why it's good for the game

Bug bad

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog

:cl: Morrow
fix: Fixed sentries firing turning off their lights
/:cl:

code/modules/defenses/sentry.dm

@@ -1,6 +1,5 @@
 #define SENTRY_FIREANGLE 135
 #define SENTRY_RANGE 5
-#define SENTRY_MUZZLELUM 3
 #define SENTRY_ENGAGED_TIMEOUT 60 SECONDS
 #define SENTRY_LOW_AMMO_TIMEOUT 20 SECONDS
 #define SENTRY_LOW_AMMO_ALERT_PERCENTAGE 0.25
@@ -336,9 +335,6 @@
 	if(isnull(angle))
 		return
 
-	SetLuminosity(SENTRY_MUZZLELUM)
-	addtimer(CALLBACK(src, TYPE_PROC_REF(/atom, SetLuminosity), -SENTRY_MUZZLELUM), 10)
-
 	var/image_layer = layer + 0.1
 	var/offset = 13
 
@@ -726,4 +722,3 @@
 
 #undef SENTRY_FIREANGLE
 #undef SENTRY_RANGE
-#undef SENTRY_MUZZLELUM