Remove .Net Foundation license header

src/Markdig/Helpers/CharHelper.cs

@@ -2,36 +2,6 @@
 // This file is licensed under the BSD-Clause 2 license. 
 // See the license.txt file in the project root for more information.
 
-
-// The IsHighSurrogate, IsLowSurrogate and ConvertToUtf32 methods are copied from
-// .Net Core source code which is under MIT license. They are copied here because
-// they don't exist in `portable40-net40+sl5+win8+wp8+wpa81`, We probably should remove them
-// once we dropped support for that target platform and use the official .Net methods.
-
-//The MIT License(MIT)
-
-//Copyright(c) .NET Foundation and Contributors
-
-//All rights reserved.
-
-//Permission is hereby granted, free of charge, to any person obtaining a copy
-//of this software and associated documentation files (the "Software"), to deal
-//in the Software without restriction, including without limitation the rights
-//to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-//copies of the Software, and to permit persons to whom the Software is
-//furnished to do so, subject to the following conditions:
-
-//The above copyright notice and this permission notice shall be included in all
-//copies or substantial portions of the Software.
-
-//THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-//IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-//FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-//AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-//LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-//OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-//SOFTWARE.
-
 using System;
 using System.Globalization;
 using System.Collections.Generic;