Remove unnecessary using directives

src/SparseTransform.CLI/CLIOptions/ConvertOptions.cs

-using System;
 using CommandLine;
 
 namespace CLIOptions;

src/SparseTransform.CLI/CLIOptions/PartitionOptions.cs

-using System;
 using CommandLine;
 
 namespace CLIOptions;

src/SparseTransform.CLI/Formats.cs

-using System;
-
 /// <summary>
 /// Permitted values for Format CLI option. Checked in Program.
 /// </summary>

src/SparseTransform.CLI/Program.cs

 using CommandLine;
-using System;
 using SparseTransform;
 using CLIOptions;
 

src/SparseTransform.Tests/Library/ConversionManagerTests.cs

 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-using DataStructures;
 using NUnit.Framework;
 using System.IO;
 

src/SparseTransform/ConversionManager.cs

 using DataStructures;
 using SparseTransform.Convert;
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 
 namespace SparseTransform
 {