[C2] Fix cache cleanup command

[ywarnier]

On /main/admin/archive_cleanup.php, the "Proceed with cleanup" button triggers an error message:

PHP Fatal error:  Uncaught ErrorException: Warning: rmdir(/var/www/host/www/var/cache//dev): Directory not empty in /var/www/host/www/public/main/inc/lib/api.lib.php:4007
Stack trace:
#0 /var/www/host/public/main/inc/lib/api.lib.php(3990): rmdirr()
#1 /var/www/host/www/public/main/admin/archive_cleanup.php(52): rmdirr()
#2 {main}\n  thrown in /var/www/host/www/public/main/inc/lib/api.lib.php on line 4007