Remove the 'header_comment' rule from php-cs-fixer

It was enforcing us to keep the Blackboard header for all files
when new ones shouldn't be subject to that copyright notice.

For the records, the new file comment should be like:

```
<?php

// This file is part of the Moodle Plugin CI package.
//
// 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/>.
```

Then, in the class PHPDocs, put the usual @copyright and @license tags.

Author: stronk7

.php-cs-fixer.php

@@ -10,16 +10,6 @@
  * License http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 
-$header = <<<'EOF'
-This file is part of the Moodle Plugin CI package.
-
-For the full copyright and license information, please view the LICENSE
-file that was distributed with this source code.
-
-Copyright (c) 2018 Blackboard Inc. (http://www.blackboard.com)
-License http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
-EOF;
-
 return (new PhpCsFixer\Config())
     ->setRiskyAllowed(true)
     ->setRules([
@@ -33,7 +23,6 @@
                                                        'expectedExceptionMessage',
                                                        'expectedExceptionMessageRegExp',
                                                    ]],
-        'header_comment'                        => ['header' => $header],
         'heredoc_to_nowdoc'                     => true,
         'no_extra_blank_lines'                  => ['tokens' => [
                                                        'break', 'continue', 'extra', 'return',