Organize RPCCommands table

Use sensible categories (overall control, P2P, blockchain/UTXO, mining, utils, wallet, wallet-enabled mining) and sort within each.

`validateaddress` was wrongly categorized under 'Mining'.
Also remove unnecessary #ifdef ENABLE_WALLET from `rpcnet.cpp`.

Functionality-neutral change (a way to display the categories in "help" would be nice as well, but is not covered in this pull).