Remove dynamic_shared_memory_type=none

PostgreSQL nowadays offers some kind of dynamic shared memory feature on
all supported platforms.  Having the choice of "none" prevents us from
relying on DSM in core features.  So this patch removes the choice of
"none".

Author: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>

doc/src/sgml/config.sgml

@@ -1662,9 +1662,9 @@ include_dir 'conf.d'
         should use.  Possible values are <literal>posix</literal> (for POSIX shared
         memory allocated using <literal>shm_open</literal>), <literal>sysv</literal>
         (for System V shared memory allocated via <literal>shmget</literal>),
-        <literal>windows</literal> (for Windows shared memory), <literal>mmap</literal>
-        (to simulate shared memory using memory-mapped files stored in the
-        data directory), and <literal>none</literal> (to disable this feature).
+        <literal>windows</literal> (for Windows shared memory),
+        and <literal>mmap</literal> (to simulate shared memory using
+        memory-mapped files stored in the data directory).
         Not all values are supported on all platforms; the first supported
         option is the default for that platform.  The use of the
         <literal>mmap</literal> option, which is not the default on any platform,

doc/src/sgml/parallel.sgml

@@ -124,14 +124,6 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
         configured via <varname>max_parallel_workers_per_gather</varname>.
       </para>
     </listitem>
-
-    <listitem>
-      <para>
-        <xref linkend="guc-dynamic-shared-memory-type"/> must be set to a
-        value other than <literal>none</literal>.  Parallel query requires dynamic
-        shared memory in order to pass data between cooperating processes.
-      </para>
-    </listitem>
   </itemizedlist>
 
   <para>