Remove test ref on UB.Tasks

Author: mthalman

src/SourceBuild/content/test/tests.proj

@@ -21,7 +21,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <ProjectReference Include="$(TasksDir)Microsoft.DotNet.UnifiedBuild.Tasks\Microsoft.DotNet.UnifiedBuild.Tasks.csproj" />
     <!-- Shared tests - applicable to unified-build and source-build. -->
     <ProjectReference Include="Microsoft.DotNet.Tests\Microsoft.DotNet.Tests.csproj" />
     <!-- Source-build specific tests. -->