gitserver: fix usage of loggers in cleanup

Created by: bobheadxi

Follow-ups to https://github.com/sourcegraph/sourcegraph/pull/36005 due to missed review comments

• Set a scoped logger for cleanup , which I renamed to cleanup.setRepoSizes (cc @sashaostrikov )
• Do not use logger.With when logging fields for a single log entry (cc @gitstart-sourcegraph )
• Duplicated logger in gitserver

Test plan

sg start