remove build workaround

.vsts.ix-shared.yml

@@ -14,13 +14,6 @@ steps:
   displayName: Set Version
   condition: eq(variables['system.pullrequest.isfork'], false)
 
-- task: DotNetCoreCLI@2
-  inputs:
-    command: build
-    projects: Ix.NET/Source/refs/**/System.Interactive*.csproj
-    arguments: -c $(BuildConfiguration)   
-  displayName: Build reference assemblies
-
 - task: DotNetCoreCLI@2
   inputs:
     command: pack