Batch writes for importmulti

When writing all of the imported data to the wallet, use a common
WalletBatch object so that batch writes are done and the writes
finish more quickly.

AddKeypoolPubkey is no longer needed so it is also removed

src/wallet/wallet.cpp

@@ -3372,13 +3372,6 @@ bool CWallet::TopUpKeyPool(unsigned int kpSize)
     return true;
 }
 
-void CWallet::AddKeypoolPubkey(const CPubKey& pubkey, const bool internal)
-{
-    WalletBatch batch(*database);
-    AddKeypoolPubkeyWithDB(pubkey, internal, batch);
-    NotifyCanGetAddressesChanged();
-}
-
 void CWallet::AddKeypoolPubkeyWithDB(const CPubKey& pubkey, const bool internal, WalletBatch& batch)
 {
     LOCK(cs_wallet);