API : Remove REPORTS_DIR and its use.

Relies on
https://github.com/silverstripe/silverstripe-framework/pull/7065

_config.php

@@ -1,6 +0,0 @@
-<?php
-
-$path = explode(DIRECTORY_SEPARATOR, dirname(__FILE__));
-$dir = $path[count($path) - 1];
-
-define('REPORTS_DIR', $dir);

code/ReportAdmin.php

@@ -65,7 +65,7 @@ class ReportAdmin extends LeftAndMain implements PermissionProvider
         // Set custom options for TinyMCE specific to ReportAdmin
         HTMLEditorConfig::get('cms')->setOption('content_css', project() . '/css/editor.css');
 
-        Requirements::javascript(REPORTS_DIR . '/javascript/ReportAdmin.js');
+        Requirements::javascript('silverstripe/reports: javascript/ReportAdmin.js');
     }
 
     /**