doc: Remove unused ids

Some reference pages contained id attributes on refname elements.
These were apparently copied around from ancient times, but they don't
serve a purpose.  FOP issues minor warnings about them.  So it's
easiest to just remove them.

doc/src/sgml/ref/clusterdb.sgml

@@ -15,7 +15,7 @@ PostgreSQL documentation
  </refmeta>
 
  <refnamediv>
-  <refname id="clusterdb">clusterdb</refname>
+  <refname>clusterdb</refname>
   <refpurpose>cluster a <productname>PostgreSQL</productname> database</refpurpose>
  </refnamediv>
 

doc/src/sgml/ref/postmaster.sgml

@@ -15,7 +15,7 @@ PostgreSQL documentation
  </refmeta>
 
  <refnamediv>
-  <refname id="postmaster-ref">postmaster</refname>
+  <refname>postmaster</refname>
   <refpurpose><productname>PostgreSQL</productname> database server</refpurpose>
  </refnamediv>
 

doc/src/sgml/ref/reindexdb.sgml

@@ -15,7 +15,7 @@ PostgreSQL documentation
  </refmeta>
 
  <refnamediv>
-  <refname id="reindexdb">reindexdb</refname>
+  <refname>reindexdb</refname>
   <refpurpose>reindex a <productname>PostgreSQL</productname> database</refpurpose>
  </refnamediv>
 

doc/src/sgml/ref/vacuumdb.sgml

@@ -15,7 +15,7 @@ PostgreSQL documentation
  </refmeta>
 
  <refnamediv>
-  <refname id="vacuumdb">vacuumdb</refname>
+  <refname>vacuumdb</refname>
   <refpurpose>garbage-collect and analyze a <productname>PostgreSQL</productname> database</refpurpose>
  </refnamediv>