Remove auxpow from CBlockIndex

Remove the auxpow field from CBlockIndex and thus also from blkindex.dat.  This field is never actually used in the current code - when other "dead" code is also removed, as done in this patch.  This reduces the storage required for blkindex.dat from 490 MiB to around 270 MiB, and increases the start-up performance dramatically for me.  (The "Loading block index..." stage takes now only 17 seconds instead of 700 seconds before!)  Also, memory usage should be reduced by around the same amount as blkindex.dat.