docs(policy): fix wrong documentation of <tcp-mss-clamp/> in `man fir…

…ewalld.policy`

This element can only be inside a rich-rule, not at policy level.
The documentation was wrong. Drop it.

Fixes: 3f93937 ('docs(rich): update docs to support tcp-mss-clamp')

https://lists.fedorahosted.org/archives/list/firewalld-users@lists.fedorahosted.org/message/ITZU3X6VRKP7ZHMYFSUDIKMP5BNQWZTR/
(cherry picked from commit 058592d)

doc/xml/policy_zone_descriptions.xml

@@ -130,25 +130,6 @@
     </variablelist>
 </refsect2>
 
-<refsect2 id="tcp-mss-clamp">
-    <title>tcp-mss-clamp</title>
-    <para>
-        Is an optional empty-element tag and can be used several times. If left empty
-	maximum segment size is set to 'pmtu'. This tag has exactly one optional attribute:
-      </para>
-      <variablelist>
-        <varlistentry>
-          <term>value="<replaceable>string</replaceable>"</term>
-          <listitem>
-            <para>
-              Value can set maximum segment size to 'pmtu' (Path Maximum Transmission Unit) or
-	      a user-defined value that is greater than or equal to 536.
-            </para>
-          </listitem>
-        </varlistentry>
-    </variablelist>
-</refsect2>
-
 <refsect2 id="masquerade">
     <title>masquerade</title>
     <para>