Move SendMoney() to rpcwallet.cpp.

This is proposed variant to #5393. SendMoney() moved from wallet.cpp/.h to rpcwallet.cpp.
RPC errors are not localized. SendMoney directly throws JSONRPCErrors instead of returning strings and guessing the proper error code. SendMoney also checks for available funds so RPC_WALLET_INSUFFICIENT_FUNDS (-6) is returned properly (fixes #3007).
Shouldn't this be RPC_WALLET_INSUFFICIENT_FUNDS too?