Remove general-testing feed from NuGet.config (#120956)

This PR removes the `general-testing` NuGet package source from the root
NuGet.config file. This feed was previously needed for a prototype
Roslyn compiler for the runtime-async effort, but is no longer required.

### Changes
- Removed the `general-testing` package source entry
- Removed the associated comment explaining its purpose

The remaining package sources (dotnet-public, dotnet-tools, dotnet-eng,
dotnet-libraries, dotnet-libraries-transport, dotnet10,
dotnet10-transport, dotnet11, and dotnet11-transport) are still
configured and functional.

### Validation
- Confirmed the XML is well-formed
- Verified no other files in the repository reference the
general-testing feed

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> Remove the following line from NuGet.config in the root of the
repository:
> 
> <add key="general-testing"
value="https://pkgs.dev.azure.com/dnceng/public/_packaging/general-testing/nuget/v3/index.json"
/>
> 
> This will remove the 'general-testing' package source from the package
sources list. No other package sources should be changed. Commit
message: "Remove general-testing feed from NuGet.config".


</details>

*This pull request was created as a result of the following prompt from
Copilot chat.*
> Remove the following line from NuGet.config in the root of the
repository:
> 
> <add key="general-testing"
value="https://pkgs.dev.azure.com/dnceng/public/_packaging/general-testing/nuget/v3/index.json"
/>
> 
> This will remove the 'general-testing' package source from the package
sources list. No other package sources should be changed. Commit
message: "Remove general-testing feed from NuGet.config".

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---

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: agocke <[email protected]>

Author: Copilot

NuGet.config

@@ -21,8 +21,6 @@
     <add key="dotnet10-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet10-transport/nuget/v3/index.json" />
     <add key="dotnet11" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet11/nuget/v3/index.json" />
     <add key="dotnet11-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet11-transport/nuget/v3/index.json" />
-    <!-- Need for prototype Roslyn compiler for runtime-async -->
-    <add key="general-testing" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/general-testing/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
     <clear />