[C#] fix: Remove public feed from nuget config (#1480)

## Linked issues

closes: #minor

## Details
* Internal security scan static analysis tool doesn't allow non-Azure
Artifcat feeds. However the specified feed will default to the public
registry.
* This feed is needed as the `Microsoft.ML.Tokenizers` library is yet to
be published to the public registry. It should be in about a week.

## Attestation Checklist

- [x] My code follows the style guidelines of this project

- I have checked for/fixed spelling, linting, and other errors
- I have commented my code for clarity
- I have made corresponding changes to the documentation (updating the
doc strings in the code is sufficient)
- My changes generate no new warnings
- I have added tests that validates my changes, and provides sufficient
test coverage. I have tested with:
  - Local testing
  - E2E testing in Teams
- New and existing unit tests pass locally with my changes

dotnet/packages/Microsoft.TeamsAI/nuget.config

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
-    <add key="dotnet-local-feed" value=" https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-libraries/nuget/v3/index.json" />
-    <add key="nuget" value="https://api.nuget.org/v3/index.json" />
+    <add key="dotnet-local-feed" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-libraries/nuget/v3/index.json" />
   </packageSources>
 </configuration>