topcat: remove svn-isms from user documents.

SUN253 and SUN256 contained $Id$ strings marking versions.
This no longer makes sense in git-land, so removed them.
topcat: remove PDF build from install target.

Building the PDF version of SUN/253 was a very slow part of the TOPCAT
build/install cycle and also sometimes failed due to a Xerces bug.
Normally, the HTML version is enough.  The PDF is now therefore not built
except by explicit request.