MDL-45130 editor_atto: Remove debugging messages for missing plugins

This is no longer required as the toolbar configuration prevents
missing plugins from being set. However, even if we do not output
any notice, we ignore missing plugins to prevent errors in case
one was uninstalled but left in the editor toolbar.

lib/editor/atto/lib.php

@@ -100,7 +100,6 @@ class atto_texteditor extends texteditor {
             foreach ($plugins as $plugin) {
                 // Do not die on missing plugin.
                 if (!core_component::get_component_directory('atto_' . $plugin))  {
-                    debugging('Missing atto plugin ' . $plugin, DEBUG_DEVELOPER);
                     continue;
                 }