Remove validator staking reward address

[brentstone]

With a new implementation of the PoS rewards that follows the F1 Fee mechanism, the validator staking reward address and its associated public key and VP code are no longer needed.

Rather than distributing rewards every epoch from the PoS account to the staking reward address, an epoched list of reward products, with values parameterized as (1 + fractional_reward) will be held in storage, while the tokens will remain held in the PoS account. The rewards are applied upon withdrawal of tokens by a validator or delegator, and the number of tokens credited will take into account the accumulated rewards.