Revert "DOC: pg_pageprep docs"

This reverts commit 42cd381.

Author: dmpgpro

doc/src/sgml/contrib.sgml

@@ -137,7 +137,6 @@ CREATE EXTENSION <replaceable>module_name</> FROM unpackaged;
  &pgcrypto;
  &pgfreespacemap;
  &pgpathman;
- &pg-pageprep;
  &pgprewarm;
  &pgquerystate;
  &pgrowlocks;

doc/src/sgml/filelist.sgml

@@ -144,7 +144,6 @@
 <!ENTITY pgcrypto        SYSTEM "pgcrypto.sgml">
 <!ENTITY pgfreespacemap  SYSTEM "pgfreespacemap.sgml">
 <!ENTITY pgpathman       SYSTEM "pgpathman.sgml">
-<!ENTITY pg-pageprep     SYSTEM "pg_pageprep.sgml">
 <!ENTITY pgprewarm       SYSTEM "pgprewarm.sgml">
 <!ENTITY pgquerystate    SYSTEM "pgquerystate.sgml">
 <!ENTITY pgrepack        SYSTEM "pgrepack.sgml">