[wallet] [rpc] Deprecate wallet 'account' API
[wallet] [rpc] Deprecate wallet 'account' API

This commit finalizes the deprecation of the wallet 'accounts' API by
removing all account arguments and return values.

RPC behaviour is slightly different if the 'accounts' or 'labels' API is
being used. Those behaviour changes are fully documented in the RPC help
text.

chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin