Drop SourceLink package reference

Per @KirillOsenkov in [this comment](dotnet/sdk#36666 (comment)), SourceLink is now built into the .NET SDK.

Author: AArnott

Directory.Packages.props

@@ -17,7 +17,4 @@
     <GlobalPackageReference Include="Nullable" Version="1.3.1" />
     <GlobalPackageReference Include="StyleCop.Analyzers.Unstable" Version="1.2.0.556" />
   </ItemGroup>
-  <ItemGroup>
-    <GlobalPackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
-  </ItemGroup>
 </Project>