contrib: remove GPL-3+ from debian/copyright

This is unused.

contrib/debian/copyright

@@ -121,17 +121,6 @@ Comment:
  You should have received a copy of the GNU General Public License along
  with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-License: GPL-3+
- Permission is granted to copy, distribute and/or modify this document
- under the terms of the GNU General Public License, Version 3 or any
- later version published by the Free Software Foundation.
-Comment:
- On Debian systems the GNU General Public License (GPL) version 3 is
- located in '/usr/share/common-licenses/GPL-3'.
- .
- You should have received a copy of the GNU General Public License along
- with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 License: public-domain
  This work is in the public domain.