cr_chains: add remove() to delete ops associated with given rmd
cr_chains: add remove() to delete ops associated with given rmd

Issue: KBFS-1474

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