Update JsonSerializer exception (dotnet#4957)

Author: gewarren

xml/System.Text.Json/JsonSerializer.xml

@@ -306,8 +306,6 @@ For more information, see [How to serialize and deserialize JSON](/dotnet/standa
 
 ]]></format>
         </remarks>
-        <exception cref="T:System.IndexOutOfRangeException">
-          .NET Core 3.0 only: <typeparamref name="TValue" /> is <see cref="T:System.Char" /> and <paramref name="json" /> is an empty string.</exception>
         <exception cref="T:System.ArgumentNullException">
           <paramref name="json" /> is <see langword="null" />.</exception>
         <exception cref="T:System.Text.Json.JsonException">The JSON is invalid.
@@ -318,11 +316,7 @@ For more information, see [How to serialize and deserialize JSON](/dotnet/standa
 
 -or-
 
-There is remaining data in the string beyond a single JSON value.
-
--or-
-
-.NET Core 3.1 and later versions only: <typeparamref name="TValue" /> is <see cref="T:System.Char" /> and <paramref name="json" /> is a null reference (<code>JsonSerializer.Deserialize&lt;char&gt;("null");</code>) or an empty string.</exception>
+There is remaining data in the string beyond a single JSON value.</exception>
         <exception cref="T:System.NotSupportedException">There is no compatible <see cref="System.Text.Json.Serialization.JsonConverter" /> for <typeparamref name="TValue" /> or its serializable members.</exception>
       </Docs>
     </Member>