Revert "MDL-39474 Library: Make a fast way to check developer debug mode"

There were multiple problems discovered.

This reverts commit 5c92e7a740.

Conflicts:
	admin/cli/install.php

admin/cli/install.php

@@ -151,7 +151,6 @@ $CFG->docroot              = 'http://docs.moodle.org';
 $CFG->running_installer    = true;
 $CFG->early_install_lang   = true;
 $CFG->ostype               = (stristr(PHP_OS, 'win') && !stristr(PHP_OS, 'darwin')) ? 'WINDOWS' : 'UNIX';
-$CFG->developerdebug       = true;
 $CFG->dboptions            = array();
 
 $parts = explode('/', str_replace('\\', '/', dirname(dirname(__FILE__))));