Add a method for removing a single wallet tx.

Also changes old ZapWalletTx function names to ZapWalletTxes to match the
command line argument (-zapwallettxes)

This could be a nice conservative option for those who have zombie transactions and need to get rid of them to get an accurate account balance. I think the user might need to perform a rescan once removing the offending tx's.

I have not tested this yet, I need to figure out how to get a testnet wallet with some zombie transactions in it.