LICENSE: drop GPL as licensing options

From now on, the only licensing option for libcoap is BSD 2-Clause.

CONTRIBUTE

@@ -6,14 +6,14 @@
 ~~~~~~~~~~~~~
 The libcoap project is a FOSS project that is dual licensed. The maintainer
 for the libcoap is Olaf Bergmann <bergmann@tzi.org>.
-Any contributions have to be made dual licensed under the terms of the
+Any contributions have to be made under the terms of the
 license
 
   * BSD 2-Clause (The BSD 2-Clause License)
 
-and
-
-  * GPL v2+ (The GNU General Public License 2.0 or later)
+Contributions made up to 2017-06-01 have been made under the dual
+license model BSD 2-Clause and GPL v2+ (The GNU General Public License
+2.0 or later).
 
 The used VCS for libcoap is Git, the main repository is living on GitHub.
 You can clone (or fork directly on GitHub) on the repository site:

COPYING

@@ -1,6 +1,5 @@
 libcoap is published as open-source software without any warranty of any kind.
 
-Use is permitted under the terms of the GNU General Public License (GPL),
-Version 2 or higher, OR the simplified BSD license.
+Use is permitted under the terms of the simplified BSD 2-Clause license.
 
-The respective license file are shipped as LICENSE.BSD and LICENSE.GPL.
+The respective license file is shipped as LICENSE.BSD.

LICENSE.BSD

@@ -1,4 +1,4 @@
-Copyright (c) 2010--2015, Olaf Bergmann
+Copyright (c) 2010--2017, Olaf Bergmann and others
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without

README.md

@@ -26,8 +26,6 @@ LICENSE INFORMATION
 ===================
 
 This library is published as open-source software without any warranty
-of any kind. Use is permitted under the terms of the GNU General
-Public License (GPL), Version 2 or higher, OR the simplified BSD
-license. Please refer to LICENSE.GPL oder LICENSE.BSD for further
-details.
+of any kind. Use is permitted under the terms of the simplified BSD
+license. Please refer to LICENSE.BSD for further details.