drop mapAlreadyAskFor

mapAlreadyAskFor doesn't have any effect unless a peer is sending invs for the
same invalid transactions more than every 2 minutes, and even in that case it
just slows down the unnecessary re-requesting.

Bitcoind never sends invs for the same tx that often (wallet rebroadcasts are at
least 5 minutes apart), and if a slightly malicious node wants to cause a lot of
unnecessary but low-cost network messages there are plenty of other ways to do
that anyway.