Remove the last vestige of server-side autocommit.

Long ago we briefly had an "autocommit" GUC that turned server-side
autocommit on and off.  That behavior was removed in 7.4 after concluding
that it broke far too much client-side logic, and making clients cope with
both behaviors was impractical.  But the GUC variable was left behind, so
as not to break any client code that might be trying to read its value.
Enough time has now passed that we should remove the GUC completely.
Whatever vestigial backwards-compatibility benefit it had is outweighed by
the risk of confusion for newbies who assume it ought to do something,
as per a recent complaint from Wolfgang Wilhelm.

In passing, adjust what seemed to me a rather confusing documentation
reference to libpq's autocommit behavior.  libpq as such knows nothing
about autocommit, so psql is probably what was meant.

doc/src/sgml/ecpg.sgml

@@ -476,7 +476,7 @@ EXEC SQL COMMIT;
    In the default mode, statements are committed only when
    <command>EXEC SQL COMMIT</command> is issued. The embedded SQL
    interface also supports autocommit of transactions (similar to
-   <application>libpq</> behavior) via the <option>-t</option>
+   <application>psql</>'s default behavior) via the <option>-t</option>
    command-line option to <command>ecpg</command> (see <xref
    linkend="app-ecpg">) or via the <literal>EXEC SQL SET AUTOCOMMIT TO
    ON</literal> statement. In autocommit mode, each command is