MDL-83704 core: Stop loading subplugins files to uninstall

The list of subplugin types was loaded from the json/php subplugins
file, but then the actual list of subplugins was loaded from the cache
in the component, rendering any benefit of using the source json/php as
pointless.

lib/adminlib.php

@@ -139,17 +139,7 @@ function uninstall_plugin($type, $name) {
     $subplugintypes = core_component::get_plugin_types_with_subplugins();
     if (isset($subplugintypes[$type])) {
         $base = core_component::get_plugin_directory($type, $name);
-
-        $subpluginsfile = "{$base}/db/subplugins.json";
-        if (file_exists($subpluginsfile)) {
-            $subplugins = (array) json_decode(file_get_contents($subpluginsfile))->plugintypes;
-        } else if (file_exists("{$base}/db/subplugins.php")) {
-            debugging('Use of subplugins.php has been deprecated. ' .
-                    'Please update your plugin to provide a subplugins.json file instead.',
-                    DEBUG_DEVELOPER);
-            $subplugins = [];
-            include("{$base}/db/subplugins.php");
-        }
+        $subplugins = \core\component::get_subplugins("{$type}_{$name}");
 
         if (!empty($subplugins)) {
             foreach (array_keys($subplugins) as $subplugintype) {