core, miner: move Backend to miner

This ensures that package core doesn't depend on package accounts and
resolves an age-old TODO.

core/types.go

@@ -19,12 +19,9 @@ package core
 import (
 	"math/big"
 
-	"github.com/ethereum/go-ethereum/accounts"
 	"github.com/ethereum/go-ethereum/core/state"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/core/vm"
-	"github.com/ethereum/go-ethereum/ethdb"
-	"github.com/ethereum/go-ethereum/event"
 )
 
 // Validator is an interface which defines the standard for block validation.
@@ -63,15 +60,3 @@ type HeaderValidator interface {
 type Processor interface {
 	Process(block *types.Block, statedb *state.StateDB, cfg vm.Config) (types.Receipts, vm.Logs, *big.Int, error)
 }
-
-// Backend is an interface defining the basic functionality for an operable node
-// with all the functionality to be a functional, valid Ethereum operator.
-//
-// TODO Remove this
-type Backend interface {
-	AccountManager() *accounts.Manager
-	BlockChain() *BlockChain
-	TxPool() *TxPool
-	ChainDb() ethdb.Database
-	EventMux() *event.TypeMux
-}