Remove redundant #nullable enable (#4275)

Content.Server/Explosion/Components/ClusterFlashComponent.cs

@@ -1,4 +1,3 @@
-#nullable enable
 using System;
 using System.Diagnostics.CodeAnalysis;
 using System.Threading.Tasks;

Content.Server/Explosion/Components/OnUseTimerTriggerComponent.cs

@@ -1,5 +1,4 @@
-#nullable enable
-using System;
+using System;
 using Content.Shared.Interaction;
 using Content.Shared.Trigger;
 using Robust.Server.GameObjects;

Content.Server/Explosion/ExplosionHelper.cs

@@ -1,4 +1,3 @@
-#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Linq;