Remove unused function ReadOwnerTxs.

Remove seemingly unused function accessing "owner" records in the
blkindex.dat database.
Remove auxpow field from blkindex.dat storage.

Disable storing of the auxpow field in blkindex.dat.  Instead, only
store there if the block has auxpow.  Whenever the auxpow itself is
needed, load it directly from the blockchain instead.
Rewrite blkindex.dat after compaction.

After removing the auxpow from the blkindex.dat database, rewrite the DB
file in order to remove "reserved" empty pages from it entirely.
Disable 'getheaders' network message.

Disable the 'getheaders' network message, since it is never actually
used and thus also not well tested.
Force fReadTransactions=true for reading blocks.

Remove fReadTransactions as optional argument in CBlock::ReadFromDisk
for CBlockIndex* arguments, since this is never actually used in the
code.  This allows to get rid of the CBlockIndex::GetBlockHeader method.
Completely remove auxpow field from CBlockIndex.

Completely remove the auxpow field from CBlockIndex, since it is
actually not necessary anywhere.
Remove redundant condition.