po: Remove wrong FSF copyright notice in Vietnamese translations

All translators that have ever worked in the Vietnamese translations:

  Phan Vinh Thinh <teppi@vnoss.org>, 2005.
  Clytie Siddall <clytie@riverland.net.au>, 2006-2010.
  Trần Ngọc Quân <vnwildman@gmail.com>, 2012-2013.

have never assigned their copyright to the FSF, they just signed the
copyright disclaimer:

  <http://translationproject.org/html/authors.html>

as such the FSF copyright notice is wrong, let's just remove it.

dselect/po/vi.po

@@ -1,5 +1,4 @@
 # Vietnamese translation for Dselect.
-# Copyright © 2006 Free Software Foundation, Inc.
 # This file is distributed under the same license as the dpkg package.
 # Phan Vinh Thinh <teppi@vnoss.org>, 2005.
 # Clytie Siddall <clytie@riverland.net.au>, 2006.

po/vi.po

@@ -1,6 +1,5 @@
 # Vietnamese translation for dpkg.
 # Bản dịch tiếng Việt dành cho dpkg.
-# Copyright © 2013 Free Software Foundation, Inc.
 # Phan Vinh Thinh <teppi@vnoss.org>, 2005.
 # Clytie Siddall <clytie@riverland.net.au>, 2006-2010.
 # Trần Ngọc Quân <vnwildman@gmail.com>, 2012-2013.