V2 Mining Contracts to only support SHARE

Currently, the mining contracts pay rewards in SHARE and USD (from Seigniorage). Some community members have expressed that this may cause longer negative rebase cycles due to slower USD dilution on the market. One solution is to add a "delete" USD function that allows any users to delete their own USD. The V2 contracts would call this function before any USD transfer, so as to only release SHARE tokens as a reward.