Doc: Remove type information for import_generated in postgres-fdw.sgml.

The type information for FDW options is only added to HEAD; remove this
from back branches.  Oversight in commit aa769f80e.

Apply the patch to v12, v13, and v14.

Discussion: https://postgr.es/m/CAPmGK14z92twaKwRoccHbbh5Va5vbRDZcTYYTx50+0JTQ8xx_g@mail.gmail.com

doc/src/sgml/postgres-fdw.sgml

@@ -556,7 +556,7 @@ OPTIONS (ADD password_required 'false');
      </listitem>
     </varlistentry>
     <varlistentry>
-     <term><literal>import_generated</literal> (<type>boolean</type>)</term>
+     <term><literal>import_generated</literal></term>
      <listitem>
       <para>
        This option controls whether column <literal>GENERATED</literal> expressions