ts: only issue DeleteRange requests when pruning if there is something to delete

[petermattis]

Discovered while investigating #10409.

tsdb.PruneTimeSeries issues a DeleteRange request for every time series name it encounters regardless of whether there is any data to delete. It seems possible to enhance tsdb.findTimeSeries to only return time series which have data that needs to be pruned. When seeking to the next time series name we could "back up" (using rocksdb.Iterator.Prev) to find the oldest key for the previous time series and determine if any data needs to be deleted.

[petermattis]

@spencerkimball didn't you fix this?

[spencerkimball]

Yes #11762