Add removeaddress for watchonly branch

[dzimbeck]

This commit to bitcoin here:

bitcoin/bitcoin@c6844c9

Shows how to remove an address for a watchonly branch. In the case of custom wallets such as BlackHalo which add a lot of watchonly addresses that are only used temporarily, it would be efficient to clean up the wallet by removing some of the unused ones. So when you get a chance, please merge this simple RPC command.