Revert "Work around implicit usings breaking change (#7745)" (#7766)

This reverts commit 5f00dd2.

Author: RussKie

src/Microsoft.DotNet.Arcade.Sdk/tools/ProjectDefaults.props

@@ -22,9 +22,6 @@
 
     <!-- Disable the message indicating we are using a preview SDK. That is understood and by design -->
     <SuppressNETCoreSdkPreviewMessage>true</SuppressNETCoreSdkPreviewMessage>
-    
-    <!-- Workaround breaking change in .NET SDK. See https://github.com/dotnet/sdk/issues/19521 -->
-    <DisableImplicitNamespaceImports>true</DisableImplicitNamespaceImports>
 
     <!-- By default do not build NuGet package for a non pkgproj project. Project may override. -->
     <IsPackable Condition="'$(MSBuildProjectExtension)' != '.pkgproj'">false</IsPackable>