MDL-76866 editor_atto: Remove old install script

This install script has not been required for a number of years because
we require the previous LTS for an LTS+1 upgrade. Once Atto was
part of core after an LTS release, it was safe to remove this file.

This is now preventing Tiny from becoming default for new installs in
some situations.

lib/editor/atto/db/install.php

@@ -1,50 +0,0 @@
-<?php
-// This file is part of Moodle - http://moodle.org/
-//
-// Moodle is free software: you can redistribute it and/or modify
-// it under the terms of the GNU General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// Moodle is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU General Public License for more details.
-//
-// You should have received a copy of the GNU General Public License
-// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
-
-/**
- * Atto upgrade script.
- *
- * @package    editor_atto
- * @copyright  2014 Damyon Wiese
- * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
- */
-
-defined('MOODLE_INTERNAL') || die();
-
-/**
- * Make the Atto the default editor for upgrades from 26.
- *
- * @return bool
- */
-function xmldb_editor_atto_install() {
-    global $CFG;
-
-    // Make Atto the default.
-    $currenteditors = $CFG->texteditors;
-    $neweditors = array();
-
-    $list = explode(',', $currenteditors);
-    array_push($neweditors, 'atto');
-    foreach ($list as $editor) {
-        if ($editor != 'atto') {
-            array_push($neweditors, $editor);
-        }
-    }
-
-    set_config('texteditors', implode(',', $neweditors));
-
-    return true;
-}