nls: Remove obsolete messages from PO files.

Suggested by Benno Schulenberg <coordinator@translationproject.org>.

* po/guix/eo.po, po/guix/pt_BR.po, po/guix/sr.po, po/guix/vi.po,
  po/packages/eo.po, po/packages/pt_BR.po, po/packages/sr.po,
  po/packages/vi.po: Remove obsolete messages.