(Bug 45854) Unset the cgroup if we don't have a memory limit

We only create the cgroup if there's a memory limit, but it
tried to always cleanup it, regardless if it was used or not,
resulting in a «/sys/fs/cgroup/memory/mediawiki/job/32466/tasks:
No such file or directory» error, when run with MW_CGROUP='mediawiki';
MW_MEM_LIMIT=0

Change-Id: I7fdb545b336ead62aa743b1bc3afdbfba2c3113f
(cherry picked from commit e77397fc1de324dfc6d7b515e9440bd564311dd9)