block_journal: fix up the func to remove the earliest entry
block_journal: fix up the func to remove the earliest entry

Suggested by @akalin-keybase.

Issue: #473

keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs