Update some comments to delete references to Visual Studio 2017

These no longer apply.

src/Workspaces/Core/Portable/Options/IDocumentOptionsProvider.cs

@@ -2,10 +2,6 @@
 
 #nullable enable
 
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -16,9 +12,7 @@ namespace Microsoft.CodeAnalysis.Options
     /// </summary>
     /// <remarks>
     /// This is passed to <see cref="IOptionService.RegisterDocumentOptionsProvider(IDocumentOptionsProvider)"/> to activate it
-    /// for a workspace. This instance then lives around for the lifetime of the workspace. This exists primarily
-    /// because right now we're keeping this support only for the Visual Studio "15" workspace, so this offers an interface
-    /// to meet in the middle.
+    /// for a workspace. This instance then lives around for the lifetime of the workspace.
     /// </remarks>
     interface IDocumentOptionsProvider
     {

src/Workspaces/Core/Portable/Options/IDocumentOptionsProviderFactory.cs

@@ -4,12 +4,9 @@
 
 namespace Microsoft.CodeAnalysis.Options
 {
-    /// <remarks>
-    /// This interface exists so the Visual Studio workspace can create the .editorconfig provider,
-    /// despite that the provider code currently lives in a Dev15-targeting assembly but
-    /// the workspace itself is still in a Dev14-targeting assembly. Once those have merged,
-    /// this can go away.
-    /// </remarks>
+    /// <summary>
+    /// A MEF-exported factory which produces <see cref="IDocumentOptionsProvider"/>s for <see cref="Workspace"/>s.
+    /// </summary>
     interface IDocumentOptionsProviderFactory
     {
         IDocumentOptionsProvider? TryCreate(Workspace workspace);