docs - remove ALTER SYSTEM reference page from docs (#7374)

* docs - remove ALTER SYSTEM reference

* Restore source for ALTER SYSTEM ref, but leave out of maps.

* Add ALTER SYSTEM to list of unsupported PostgreSQL features

gpdb-doc/dita/ref_guide/GPReferenceSQLSummaryLOP.xml

@@ -184,13 +184,6 @@
 		<codeblock conref="sql_commands/ALTER_SERVER.xml#topic1/sql_command_synopsis"/>
 		<p otherprops="op-html">See <xref href="sql_commands/ALTER_SERVER.xml"/> for more
 			information.</p>
-		<section id="section_wmv_xwd_1hb">
-			<title>ALTER SYSTEM</title>
-		</section>
-		<p conref="sql_commands/ALTER_SYSTEM.xml#topic1/sql_command_desc"/>
-		<codeblock conref="sql_commands/ALTER_SYSTEM.xml#topic1/sql_command_synopsis"/>
-		<p otherprops="op-html">See <xref href="sql_commands/ALTER_SYSTEM.xml"/> for more
-			information.</p>
 		<section id="ae1903563">
 			<title>ALTER TABLE</title>
 		</section>

gpdb-doc/dita/ref_guide/feature_summary.xml

@@ -311,6 +311,11 @@ NEXT 10 ROWS ONLY; </codeblock><p>Greenplum
               <entry colname="col2">YES</entry>
               <entry colname="col3"/>
             </row>
+            <row>
+              <entry><codeph>ALTER SYSTEM</codeph></entry>
+              <entry><b>NO</b></entry>
+              <entry/>
+            </row>
             <row>
               <entry colname="col1"><codeph>ALTER TABLE</codeph></entry>
               <entry colname="col2">YES</entry>

gpdb-doc/dita/ref_guide/ref_guide.ditamap

@@ -33,7 +33,6 @@
 			<topicref href="sql_commands/ALTER_SCHEMA.xml"/>
 			<topicref href="sql_commands/ALTER_SEQUENCE.xml"/>
 			<topicref href="sql_commands/ALTER_SERVER.xml"/>
-      <topicref href="sql_commands/ALTER_SYSTEM.xml"/>
 			<topicref href="sql_commands/ALTER_TABLE.xml"/>
 			<topicref href="sql_commands/ALTER_TABLESPACE.xml"/>
 			<topicref href="sql_commands/ALTER_TEXT_SEARCH_CONFIGURATION.xml"/>

gpdb-doc/dita/ref_guide/sql_commands/sql_ref.xml

@@ -77,9 +77,6 @@
       <li id="ad142566">
         <xref href="./ALTER_SERVER.xml#topic1" type="topic" format="dita"/>
       </li>
-      <li id="altersystem">
-        <xref href="./ALTER_SYSTEM.xml#topic1" type="topic" format="dita"/>
-      </li>
       <li id="ad140304">
         <xref href="./ALTER_TABLE.xml#topic1" type="topic" format="dita"/>
       </li>