remove needless getChunk() wrapper
remove needless getChunk() wrapper

it can be trivially demonstrated that
oldestPos/CurrentChunkPos/newestPos are always valid.

Also, we've never seen this panic in production in more than 3 years.

raintank/metrictank,replay/metrictank,replay/metrictank,raintank/raintank-metric,raintank/metrictank,raintank/raintank-metric,raintank/metrictank,raintank/raintank-metric,replay/metrictank,raintank/metrictank,replay/metrictank