MDL-76052 webservice_xmlrpc: Remove it completely from core

Normal removal procedure:
  - Remove the plugin completely from core.
  - Document it in the webservices upgrade.txt file.
  - Add a core upgrade step to proceed to remove any configuration
    if the plugin has not been re-installed manually.

Plus:
  - Remove a few remaining uses in the hub/sites registration scripts,
    that were moved from xmlrpc to hand.made rest calls by MDL-31436
    (Moodle 3.4.1 and up) and never removed then.
  - Remove the php-xmlrpc extension as a recommendation in composer.
  - Remove "xmlrpc" from various comments, trivial cleanup.

Note:
  - While working on this MDL-76078 has been created about to
    fix a serious design problem detected (it does not affect
    functionality). That's out from this issue scope.

lib/classes/hub/api.php

@@ -25,7 +25,6 @@
 namespace core\hub;
 defined('MOODLE_INTERNAL') || die();
 
-use webservice_xmlrpc_client;
 use moodle_exception;
 use curl;
 use stdClass;
@@ -370,4 +369,4 @@ class api {
     public static function upload_course_backup($hubcourseid, \stored_file $backupfile) {
         debugging("This function has been deprecated as part of the Moodle.net sunsetting process.");
     }
-}
+}

lib/classes/plugin_manager.php

@@ -1745,7 +1745,7 @@ class core_plugin_manager {
                 'binarius', 'boxxie', 'brick', 'canvas', 'formal_white', 'formfactor', 'fusion', 'leatherbound',
                 'magazine', 'mymobile', 'nimble', 'nonzero', 'overlay', 'serenity', 'sky_high', 'splash',
                 'standard', 'standardold'),
-            'webservice' => array('amf'),
+            'webservice' => array('amf', 'xmlrpc'),
         );
 
         if (!isset($plugins[$type])) {
@@ -2047,7 +2047,7 @@ class core_plugin_manager {
             ),
 
             'webservice' => array(
-                'rest', 'soap', 'xmlrpc'
+                'rest', 'soap'
             ),
 
             'workshopallocation' => array(

lib/classes/plugininfo/webservice.php

@@ -116,10 +116,6 @@ class webservice extends base {
     }
 
     public function is_uninstall_allowed() {
-        // The xmlrpc plugin contains webservice_xmlrpc_client (used by core).
-        if ($this->name == 'xmlrpc') {
-            return false;
-        }
         return true;
     }
 }

lib/db/upgrade.php

@@ -3005,5 +3005,31 @@ privatefiles,moodle|/user/files.php';
         upgrade_main_savepoint(true, 2022102800.01);
     }
 
+    if ($oldversion < 2022110600.00) {
+        // If webservice_xmlrpc isn't any longer installed, remove its configuration,
+        // capabilities and presence in other settings.
+        if (!file_exists($CFG->dirroot . '/webservice/xmlrpc/version.php')) {
+            // No DB structures to delete in this plugin.
+
+            // Remove capabilities.
+            capabilities_cleanup('webservice_xmlrpc');
+
+            // Remove own configuration.
+            unset_all_config_for_plugin('webservice_xmlrpc');
+
+            // Remove it from the enabled protocols if it was there.
+            $protos = get_config('core', 'webserviceprotocols');
+            $protoarr = explode(',', $protos);
+            $protoarr = array_filter($protoarr, function($ele) {
+                return trim($ele) !== 'xmlrpc';
+            });
+            $protos = implode(',', $protoarr);
+            set_config('webserviceprotocols', $protos);
+        }
+
+        // Main savepoint reached.
+        upgrade_main_savepoint(true, 2022110600.00);
+    }
+
     return true;
 }