metrics: drop the 3s sleeps on docker stops

[grahamwhaley]

Over on #476 we introduced serialisation and an extra sleep when shutting down multiple containers as it was showing up a lockup condition. Check that lockup has gone, and revert!

To check the problem has gone, recommendation is:

• run the metrics PR script under a VM
• run the mega death soak test, and see what happens when it tries to remove 110 nginx's

[grahamwhaley]

Also related, #563

[GabyCT]

Closing this issue as it has been fixed