Remove unnecesary using; bump lib versions

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/Microsoft.OpenApi.Readers.csproj

@@ -10,7 +10,7 @@
         <Company>Microsoft</Company>
         <Title>Microsoft.OpenApi.Readers</Title>
         <PackageId>Microsoft.OpenApi.Readers</PackageId>
-        <Version>1.6.4-preview3</Version>
+        <Version>1.6.4-preview4</Version>
         <Description>OpenAPI.NET Readers for JSON and YAML documents</Description>
         <Copyright>© Microsoft Corporation. All rights reserved.</Copyright>
         <PackageTags>OpenAPI .NET</PackageTags>

src/Microsoft.OpenApi/Microsoft.OpenApi.csproj

@@ -11,7 +11,7 @@
         <Company>Microsoft</Company>
         <Title>Microsoft.OpenApi</Title>
         <PackageId>Microsoft.OpenApi</PackageId>
-        <Version>1.6.4-preview3</Version>
+        <Version>1.6.4-preview4</Version>
         <Description>.NET models with JSON and YAML writers for OpenAPI specification</Description>
         <Copyright>© Microsoft Corporation. All rights reserved.</Copyright>
         <PackageTags>OpenAPI .NET</PackageTags>

src/Microsoft.OpenApi/Models/OpenApiOperation.cs

@@ -4,8 +4,6 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using Microsoft.OpenApi.Any;
-using Microsoft.OpenApi.Helpers;
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Writers;