Fixes notifications emitting light (#6088)

# About the pull request
Fixes #5985, albeit there might be a better approach.

# Explain why it's good for the game
bug fix, tested it with the steps listed in the issue. Worked as far as
I could tell.

# Changelog
:cl:
fix: Notifications should no longer emit light.
/:cl:

---------

Co-authored-by: DOOM <N/A>

code/modules/mob/mob_helpers.dm

@@ -605,7 +605,7 @@ GLOBAL_LIST_INIT(limb_types_by_name, list(
 
 	alert_overlay.layer = FLOAT_LAYER
 	alert_overlay.plane = FLOAT_PLANE
-
+	alert_overlay.underlays.Cut()
 	screen_alert.overlays += alert_overlay
 
 /mob/proc/reset_lighting_alpha()