Remove redundant #nullable enable (#4275)

Content.Server/DoAfter/DoAfter.cs

@@ -1,5 +1,4 @@
-#nullable enable
-using System;
+using System;
 using System.Threading.Tasks;
 using Content.Server.Hands.Components;
 using Content.Server.Items;

Content.Server/DoAfter/DoAfterComponent.cs

@@ -1,5 +1,4 @@
-#nullable enable
-using System.Collections.Generic;
+using System.Collections.Generic;
 using Content.Shared.Damage;
 using Content.Shared.DoAfter;
 using Robust.Shared.GameObjects;

Content.Server/DoAfter/DoAfterEventArgs.cs

@@ -1,5 +1,4 @@
-#nullable enable
-using System;
+using System;
 using System.Threading;
 using Content.Shared.Interaction.Helpers;
 using Content.Shared.Physics;

Content.Server/DoAfter/DoAfterSystem.cs

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