Eliminate duplicate code for getNonRootIndexedKey in HFileBlockIndex

The code to get the midkey by midLeafBlock in HFileBlockIndex.CellBasedKeyBlockIndexReader.midkey method is almost the same as HFileBlockIndex.BlockIndexReader.getNonRootIndexedKey, the code is somewhat complicated, we could use the HFileBlockIndex.BlockIndexReader.getNonRootIndexedKey directly in HFileBlockIndex.CellBasedKeyBlockIndexReader.midkey.