The unmount+full-teardown of EC shards was duplicated: the plugin-worker EC
task had unmountAndDeleteEcShards and the shell had unmountAndDeleteEcShardsQuiet,
byte-identical apart from a fence parameter and a sentinel error. That
duplication is how the teardown fence semantics drifted between the two paths.
Distribute, mount and verify already live in weed/storage/erasure_coding and are
shared by both callers; move the teardown there too, as UnmountAndDeleteEcShards
plus the shared ErrFullTeardownNotAcked sentinel. Both paths now call the one
function, so the fence semantics cannot diverge again. The shell keeps a thin
type-converting wrapper and aliases the sentinel; behavior is unchanged.