Removal from streamReaders can be done in finally

As part of the fix for https://issues.apache.org/jira/browse/HBASE-21551 we removed the storeReaders from the set inside the try block.
The code there seems to catch IOException. But if you drill down and see how the actual readers are closed - FSDataInputStreamWrapper#close() uses IOUtils.closeQuietly where we swallow the IOException (So ideally IOException will not be thrown). But there are cases we endup getting other type of RuntimeExceptions which may fail the close() and we endup not removing the storeReader from the Set. So it is safe to always remove it in finally.