Remove second public ctor from TagHelperDescriptor.

- TagHelperDescriptors are serialized/deserialized regularly, they can't have more than 1 ctor.

src/Microsoft.AspNet.Razor.Runtime/TagHelpers/TagHelperDescriptorFactory.cs

@@ -41,10 +41,12 @@ namespace Microsoft.AspNet.Razor.Runtime.TagHelpers
             var attributeDescriptors = GetAttributeDescriptors(type);
 
             return tagNames.Select(tagName =>
-                new TagHelperDescriptor(tagName,
-                                        typeName,
-                                        assemblyName,
-                                        attributeDescriptors));
+                new TagHelperDescriptor(
+                    prefix: string.Empty,
+                    tagName: tagName,
+                    typeName: typeName,
+                    assemblyName: assemblyName,
+                    attributes: attributeDescriptors));
         }
 
         private static IEnumerable<string> GetTagNames(Type tagHelperType)

src/Microsoft.AspNet.Razor/TagHelpers/TagHelperDescriptor.cs

@@ -26,17 +26,9 @@ namespace Microsoft.AspNet.Razor.TagHelpers
         }
 
         /// <summary>
-        /// Instantiates a new instance of the <see cref="TagHelperDescriptor"/> class with the given
-        /// <paramref name="attributes"/>.
+        /// Internal for testing.
         /// </summary>
-        /// <param name="tagName">The tag name that the tag helper targets. '*' indicates a catch-all
-        /// <see cref="TagHelperDescriptor"/> which applies to every HTML tag.</param>
-        /// <param name="typeName">The full name of the tag helper class.</param>
-        /// <param name="assemblyName">The name of the assembly containing the tag helper class.</param>
-        /// <param name="attributes">
-        /// The <see cref="TagHelperAttributeDescriptor"/>s to request from the HTML tag.
-        /// </param>
-        public TagHelperDescriptor(
+        internal TagHelperDescriptor(
             [NotNull] string tagName,
             [NotNull] string typeName,
             [NotNull] string assemblyName,