Removed unnecessary using System declaratives
Also so unnecessary pragma disable warnings
MODIFY
src/Chapter04/Listing04.24.WithCodeBlock.cs
src/Chapter04/Listing04.24.WithCodeBlock.cs
// Justification: Checking for null isn't discussed yet.
#pragma warning disable CS8604 // Possible null reference argument
-using System;
namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter04.Listing04_24
{

MODIFY
src/Chapter04/Listing04.25.RelyingOnIndentation.cs
src/Chapter04/Listing04.25.RelyingOnIndentation.cs
-using System;
-
namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter04.Listing04_25
{
public class Program

MODIFY
src/Chapter04/Listing04.26.IfStatementWithCurlyBraces.cs
src/Chapter04/Listing04.26.IfStatementWithCurlyBraces.cs
-using System;
-
namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter04.Listing04_26
{
public class Program

MODIFY
src/Chapter04/Listing04.36.NullCoalescingOperator.cs
src/Chapter04/Listing04.36.NullCoalescingOperator.cs
-#pragma warning disable CS0168 // Variable is declared but never used
-#pragma warning disable IDE0059 // Unnecessary assignment of a value
-
-using System;
-
namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter04.Listing04_36
{
public class Program

Removed unnecessary using System declaratives

Also so unnecessary pragma disable warnings