Revert "Fix build error?"

This reverts commit 7d35fca.

Author: chsienki

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Formatting/FormattingContext.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;