Remove UseRoslynCompilers prop and unify roslyn import

UseRoslynCompilers was introduced in buildtools by
dotnet/buildtools#947, with different
behaviors on windows/unix. It was removed by
dotnet/buildtools#1974, so we can unify our
roslyn imports now.

Author: sbomer

tests/dir.props

@@ -103,8 +103,7 @@
   </PropertyGroup>
 
   <!-- Use Roslyn Compilers to build -->
-  <Import Project="$(RoslynPropsFile)" Condition="'$(RunningOnUnix)'!='true' and Exists('$(RoslynPropsFile)') and '$(UseRoslynCompilers)'!='false' and '$(RoslynIncompatibleMsbuildVersion)' != 'true'" />
-  <Import Project="$(RoslynPropsFile)" Condition="'$(RunningOnUnix)'=='true' and Exists('$(RoslynPropsFile)') and '$(RoslynIncompatibleMsbuildVersion)' != 'true'" />
+  <Import Project="$(RoslynPropsFile)" Condition="Exists('$(RoslynPropsFile)') and '$(RoslynIncompatibleMsbuildVersion)' != 'true'" />
 
   <PropertyGroup Condition="'$(BuildAllProjects)'=='true'">
     <!-- When we do a traversal build we get all packages up front, don't restore them again -->