fix checkpoint timer (#634)

Summary:
If arg.keep_interval_updates or args.keep_last_epochs > 0, `checkpoints` would refer to a list of checkpoint files to be removed, which can be empty. So moved the logging code to the right position.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/634

Differential Revision: D14933655

Pulled By: myleott

fbshipit-source-id: 68182ee99d9701e1536833d31e0a7c5d2eb2d679