Remove rarely used public obsolete public API.

- The `DocumentWriter` case should never be called because you need to override a lot of the internals of the RazorEngine to get to the document writer. Therefore, little harm in removing the method.
- `ProjectChangeEventArgs` used to be uised by the LiveShare extension. We've since taken ownership of those bits and no longer need to maintain those constructors.
- `DefaultTagHelperDescriptorProvider.DesignTime` didn't actually do anything. Removing since the code was truly dead and did nothing functionally.

dotnet/aspnetcore-tooling#7146
\n\nCommit migrated from ba6450a695

src/Razor/Microsoft.AspNetCore.Razor.Language/src/CodeGeneration/DocumentWriter.cs

@@ -24,23 +24,6 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             return new DefaultDocumentWriter(codeTarget, options);
         }
 
-        [EditorBrowsable(EditorBrowsableState.Never)]
-        [Obsolete("This method was intended to be static, use CreateDefault instead.")]
-        public DocumentWriter Create(CodeTarget codeTarget, RazorCodeGenerationOptions options)
-        {
-            if (codeTarget == null)
-            {
-                throw new ArgumentNullException(nameof(codeTarget));
-            }
-
-            if (options == null)
-            {
-                throw new ArgumentNullException(nameof(options));
-            }
-
-            return new DefaultDocumentWriter(codeTarget, options);
-        }
-
         public abstract RazorCSharpDocument WriteDocument(RazorCodeDocument codeDocument, DocumentIntermediateNode documentNode);
     }
 }

src/Razor/Microsoft.CodeAnalysis.Razor/src/DefaultTagHelperDescriptorProvider.cs

@@ -9,12 +9,6 @@ namespace Microsoft.CodeAnalysis.Razor
 {
     public sealed class DefaultTagHelperDescriptorProvider : RazorEngineFeatureBase, ITagHelperDescriptorProvider
     {
-        [Obsolete(
-            "This property is obsolete will not be honored. Documentation will be included if " +
-            "TagHelperDescriptorProviderContext.IncludeDocumentation is set to true. Hidden tag helpers will" +
-            "be excluded from the results if TagHelperDescriptorProviderContext.ExcludeHidden is set to true.")]
-        public bool DesignTime { get; set; }
-
         public int Order { get; set; }
 
         public void Execute(TagHelperDescriptorProviderContext context)