Remove redundant #nullable enable (#4275)

Content.Shared/Module/IModuleManager.cs

@@ -1,5 +1,4 @@
-#nullable enable
-namespace Content.Shared.Module
+namespace Content.Shared.Module
 {
     /// <summary>
     /// Provides a simple way to check whether calling code is being run by

Content.Shared/Module/SharedModuleTestingCallbacks.cs

@@ -1,4 +1,3 @@
-#nullable enable
 using System;
 using Robust.Shared.ContentPack;