Remove unnecessary Microsoft.Net.Compilers.Toolset package (#1448)

Author: stephentoub

dotnet/Directory.Packages.props

@@ -105,7 +105,6 @@
     <!-- Symbols -->
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
     <!-- Toolset -->
-    <PackageVersion Include="Microsoft.Net.Compilers.Toolset" Version="4.14.0" />
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp" Version="4.14.0" />
     <PackageVersion Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="9.0.0" />
     <PackageReference Include="Microsoft.CodeAnalysis.NetAnalyzers">