Upgrade manuals' licenses to GFDL 1.3 and remove cover texts.

This commit changes the licenses for the PSPP manual from GFDL version 1.1
(or later) to GFDL version 1.3 (or later).

Perhaps more importantly, it removes the Front-Cover Text and Back-Cover
Text, as permitted by the GNU guide for maintainers:

      Please adjust the list of invariant sections as appropriate for your
   manual.  If there are none, then say "with no Invariant Sections".  If
   your manual is not published by the FSF, and under 400 pages, you can
   omit both cover texts.