startIndexReopenThread() no longer necessary

[vladak]

startIndexReopenThread() that calls maybeRefreshIndexSearchers() periodically is no longer needed thanks to the changes for #1187.

[vladak]

This might come handy for almost-atomic reindex - the index can stay the same and only historyindex/xref/source are shuffled around. I.e. it would be unwanted if the index was reopened because of the timer when the reindex was still in progress. It might cause no harm however it is not useful and current code is not ready for indexRefreshPeriod value of 0.