Remove automatic update of default key

This equalizes the behavior between the GUI and bitcoind and removes the last usage of fHaveGui (the other is removed in #3072).

The commit doesn't completely remove default key functionality but removes the code that sets a new default key after a transaction when it received coins.
Completely removing default key functionality is not that easy because it is used as stopgap in a few places

- https://github.com/bitcoin/bitcoin/blob/master/src/wallet.cpp#L1453 (in `CWallet::LoadWallet`) to determine whether this is the first run with this wallet.

- https://github.com/bitcoin/bitcoin/blob/master/src/wallet.cpp#L1837 (in `CReserveKey::GetReservedKey`) to return something when the keys are exhausted and keypool should be topped up