Remove copyright entry for no longer available plantuml extension

The PlantUML extension is now maintained in a separate git repository.

Author: travkin79

LICENSE.txt

@@ -4,11 +4,6 @@ All rights reserved.
 Copyright (c) 2016-2018, Vladimir Schneider,
 All rights reserved.
 
-Some flexmark extensions are under
-Copyright (c) 2022-2024, Advantest Europe GmbH.
-All rights reserved.
-
-
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are met:
 

README.md

@@ -605,10 +605,6 @@ Copyright (c) 2015-2016 Atlassian and others.
 
 Copyright (c) 2016-2023, Vladimir Schneider.
 
-Some flexmark extensions are under
-Copyright (c) 2022-2024, Advantest Europe GmbH.
-
-
 BSD (2-clause) licensed, see [LICENSE.txt] file.
 
 [Admonition Extension, Material for MkDocs]: https://squidfunk.github.io/mkdocs-material/reference/admonitions/