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.

Note that the string has been removed, and not deprecated, because it's
a non-generic string, not belonging to core/moodle main lang file, and
hardly reused ever. That fits with the allowed deletions, not requiring
any deprecation.

Of course, the lang removal only has been applied to master (4.1dev).
Older branches still keep it and will be used when checking < 4.1
upgradability.

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" />

lang/en/admin.php

@@ -1543,7 +1543,6 @@
 $string['warningiconvbuggy'] = 'Your version of the iconv library does not support the //IGNORE modifier. You should install the mbstring extension which can be used instead for cleaning strings containing invalid UTF-8 characters.';
 $string['webproxy'] = 'Web proxy';
 $string['webproxyinfo'] = 'Fill in the following options if your Moodle server cannot access the internet directly. Internet access is required for the download of environment data, language packs, RSS feeds, timezones, etc.<br /><em>The PHP cURL extension is highly recommended.</em>';
-$string['xmlrpcrecommended'] = 'The XMLRPC extension is useful for web services and Moodle networking.';
 $string['xmlrpcmaharaenabled'] = 'It has been detected that the Mahara ePortfolio is enabled on your site. This feature relies on the PHP XML-RPC extension which is no longer maintained by PHP.';
 $string['xmlrpcmnetauthenticationenabled'] = 'It has been detected that the MNet authentication is enabled on your site. This feature relies on the PHP XML-RPC extension which is no longer maintained by PHP.';
 $string['xmlrpcmnetenabled'] = 'It has been detected that the Moodle Networking is enabled on your site. This feature relies on the PHP XML-RPC extension which is no longer maintained by PHP.';