Delete the EC placement package and the dead encode planner code
Now that encode (and repair) place via ecbalancer.Place, nothing uses the
erasure_coding/placement package or the EC-only planner machinery
(ecPlacementPlanner, diskInfosToCandidates, calculateECScoreCandidate,
distributeECShards) in detection.go. Removes them and the package, along with the
planner-direct unit tests.