C#: Remove test explorer recommendations (superseded by C# dev kit)

Author: hvitved

csharp/.vscode/extensions.json

@@ -1,9 +1,7 @@
 {
     "recommendations": [
-        "formulahendry.dotnet-test-explorer",
         "gaoshan0621.csharp-format-usings",
         "github.vscode-codeql",
-        "hbenl.vscode-test-explorer",
         "ms-dotnettools.csharp",
         "ms-dotnettools.csdevkit"
     ],