MDL-76054 environment: Remove the php-xmlrpc extension check

For Moodle 4.1 and up, the php-xmlrpc is not needed anymore:
- All the MNet stuff has been moved to use php library (MDL-76055).
- The webservice/xmlrpc has been moved from core to contrib (MDL-76052).

So we just remove the check here. Starting with 4.1, it's not
needed for any core functionality.

No lang string changes in this branch, only in master (4.1dev).

admin/environment.xml

@@ -3781,11 +3781,6 @@
           <ON_CHECK message="tokenizerrecommended" />
         </FEEDBACK>
       </PHP_EXTENSION>
-      <PHP_EXTENSION name="xmlrpc" level="optional">
-        <FEEDBACK>
-          <ON_CHECK message="xmlrpcrecommended" />
-        </FEEDBACK>
-      </PHP_EXTENSION>
       <PHP_EXTENSION name="soap" level="optional">
         <FEEDBACK>
           <ON_CHECK message="soaprecommended" />