md_ops: simplify validation code
md_ops: simplify validation code

getForHandle should always work, regardless of being logged in or not.
Plus `GetLatestHandleForTLF` won't work for implicit team TLFs since
the server won't have the ID.

Issue: KBFS-3045

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