Flatten the type hierarchy

Nothing is using UnresolvedRQNode directly, so we can just flatten the
type hierarchy a bit.

src/Features/Core/Portable/RQName/Nodes/RQNode.cs

@@ -9,8 +9,7 @@
 
 namespace Microsoft.CodeAnalysis.Features.RQName.Nodes
 {
-    // an unresolved but parsed representation of an RQ Name
-    internal abstract class UnresolvedRQNode
+    internal abstract class RQNode
     {
         protected abstract string RQKeyword { get; }
 

src/Features/Core/Portable/RQName/Nodes/ResolvedRQNode.cs

-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-// See the LICENSE file in the project root for more information.
-
-#nullable enable
-
-namespace Microsoft.CodeAnalysis.Features.RQName.Nodes
-{
-    internal abstract class RQNode : UnresolvedRQNode
-    {
-    }
-}

src/Features/Core/Portable/RQName/RQNodeBuilder.cs

@@ -18,7 +18,7 @@ internal static class RQNodeBuilder
         /// Builds the RQName for a given symbol.
         /// </summary>
         /// <returns>The node if it could be created, otherwise null</returns>
-        public static UnresolvedRQNode? Build(ISymbol symbol)
+        public static RQNode? Build(ISymbol symbol)
             => symbol switch
             {
                 INamespaceSymbol namespaceSymbol => BuildNamespace(namespaceSymbol),