Remove hashagg_avoid_disk_plan GUC.

Note: This GUC was originally named enable_hashagg_disk when it appeared
in commit 1f39bce0, which added disk-based hash aggregation.  It was
subsequently renamed in commit 92c58fd9.

Author: Peter Geoghegan
Reviewed-By: Jeff Davis, Álvaro Herrera
Discussion: https://postgr.es/m/9d9d1e1252a52ea1bad84ea40dbebfd54e672a0f.camel%40j-davis.com
Backpatch: 13-, where disk-based hash aggregation was introduced.

doc/src/sgml/config.sgml

@@ -4813,23 +4813,6 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-hashagg-avoid-disk-plan" xreflabel="hashagg_avoid_disk_plan">
-      <term><varname>hashagg_avoid_disk_plan</varname> (<type>boolean</type>)
-      <indexterm>
-       <primary><varname>hashagg_avoid_disk_plan</varname> configuration parameter</primary>
-      </indexterm>
-      </term>
-      <listitem>
-       <para>
-        If set to <literal>on</literal>, causes the planner to avoid choosing
-        hashed aggregation plans that are expected to use the disk. If hashed
-        aggregation is chosen, it may still require the use of disk at
-        execution time, even if this parameter is enabled. The default is
-        <literal>off</literal>.
-       </para>
-      </listitem>
-     </varlistentry>
-
      </variablelist>
      </sect2>
      <sect2 id="runtime-config-query-constants">

doc/src/sgml/release-13.sgml

@@ -627,8 +627,7 @@ Author: Jeff Davis <jdavis@postgresql.org>
 
        <para>
         Previously, hash aggregation was avoided if it was expected to use
-        more than <xref linkend="guc-work-mem"/> memory.  This is controlled
-        by <xref linkend="guc-hashagg-avoid-disk-plan"/>.
+        more than <xref linkend="guc-work-mem"/> memory.
        </para>
       </listitem>