kbfs_ops: let FBO remove itself from favorites if possible
kbfs_ops: let FBO remove itself from favorites if possible

Issue: KBFS-848

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