Remove Microsoft.SourceLink.GitHub PackageReference (#19744)

SourceLink comes with the SDK now, and having the old package has an incremental build bug, see:
https://github.com/dotnet/sdk/issues/36666#issuecomment-2162173453

build/SourceLink.props

@@ -14,10 +14,6 @@
     <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
   </PropertyGroup>
   
-  <ItemGroup>
-    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All"/>
-  </ItemGroup>
-
   <!-- Workaround for https://github.com/dotnet/sdk/issues/11105 -->
   <ItemGroup>
     <SourceRoot Include="$([MSBuild]::EnsureTrailingSlash($(NuGetPackageRoot)))" Condition="'$(NuGetPackageRoot)' != ''" />