Remove introductory text from license file

The project is licensed under the GNU General Public License v3.0. In order for tools to be able to automatically
identify the license type, it is essential for the license file to contain only the exact text of the license:

https://choosealicense.com/licenses/gpl-3.0/#

LICENSE.txt

@@ -1,17 +1,3 @@
-This file includes licensing information for Arduino GitHub Actions.
-
-Copyright (c) 2019 ARDUINO SA
-
-The software is released under the GNU General Public License, which covers the main body
-of the [insert app name] code. The terms of this license can be found at:
-https://www.gnu.org/licenses/gpl-3.0.en.html
-
-You can be released from the requirements of the above licenses by purchasing
-a commercial license. Buying such a license is mandatory if you want to modify or
-otherwise use the software for commercial activities involving the Arduino
-software without disclosing the source code of your own applications. To purchase
-a commercial license, send an email to [email protected]
-
                     GNU GENERAL PUBLIC LICENSE
                        Version 3, 29 June 2007