Don't generate plain-text HISTORY and src/test/regress/README anymore.

Providing this information as plain text was doubtless worth the trouble
ten years ago, but it seems likely that hardly anyone reads it in this
format anymore.  And the effort required to maintain these files (in the
form of extra-complex markup rules in the relevant parts of the SGML
documentation) is significant.  So, let's stop doing that and rely solely
on the other documentation formats.

Per discussion, the plain-text INSTALL instructions might still be worth
their keep, so we continue to generate that file.

Rather than remove HISTORY and src/test/regress/README from distribution
tarballs entirely, replace them with simple stub files that tell the reader
where to find the relevant documentation.  This is mainly to avoid possibly
breaking packaging recipes that expect these files to exist.

Back-patch to all supported branches, because simplifying the markup
requirements for release notes won't help much unless we do it in all
branches.

README

@@ -17,8 +17,7 @@ See the file INSTALL for instructions on how to build and install
 PostgreSQL.  That file also lists supported operating systems and
 hardware platforms and contains information regarding any other
 software packages that are required to build or run the PostgreSQL
-system.  Changes between all PostgreSQL releases are recorded in the
-file HISTORY.  Copyright and license information can be found in the
+system.  Copyright and license information can be found in the
 file COPYRIGHT.  A comprehensive documentation set is included in this
 distribution; it can be read as described in the installation
 instructions.