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/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(