[MdR] remove obsolete LICENSE file, from when the plug-in wasn't distributed

src/plugins/markdown-report/LICENSE

-Files in this directory are part of the MarkDown Report Frama-C plug-in.
-
-Copyright (C) 2007-2020
-  CEA (Commissariat à l'énergie atomique et aux énergies
-       alternatives)
-
-All rights reserved.
-Contact CEA LIST for licensing.
-