dotnet#1638: don't use async when nothing is awaited.

src/Microsoft.CodeQuality.Analyzers/Core/ApiDesignGuidelines/StringBuilderAppendShouldNotTakeSubstring.Fixer.cs

@@ -29,7 +29,7 @@ public sealed override FixAllProvider GetFixAllProvider()
             return WellKnownFixAllProviders.BatchFixer;
         }
 
-        public sealed override async Task RegisterCodeFixesAsync(CodeFixContext context)
+        public sealed override Task RegisterCodeFixesAsync(CodeFixContext context)
         {
             foreach (var diagnostic in context.Diagnostics)
             {
@@ -52,6 +52,8 @@ public sealed override async Task RegisterCodeFixesAsync(CodeFixContext context)
                             diagnostic);
                 }
             }
+
+            return Task.CompletedTask;
         }
 
         private static async Task<Document> FixCodeOneParameter(