Simplify OUSD

At the moment, OUSD is backed by a basket of assets: USDT, USDS and USDC.

The main objective of this proposal is to simplify OUSD by making it backed by just USDC and removing the other two assets. We would also aim to add new USDC strategies for OUSD that will generate a higher yield than the current strategies. This will eventually make the contract simpler and it's behavior more inline with our other OTokens.

Removing USDT and USDS from the OUSD Vault will not require us to modify any of the existing contracts or redeploy or upgrade them. It would only need a new on-chain governance proposal to remove those two assets (which will go through the timelock). The on-chain proposal should also remove any strategies that are currently used for USDT and USDS. These include: Morpho Gauntlet strategy for USDT and Sky Saving Rate strategy for USDS.

The Guardian (2/8 Multisig), however, will have to swap out all the USDT and USDS held in the OUSD Vault to USDC before the on-chian proposal is executed. The Guardian will also be responsible for allocating the swapped out assets to strategies as they deem fit.

New strategies for OUSD will be added via on-chain governance soon. And we will consider simplifying the OUSD code further in the future to optimize it for gas consumption and reducing the code complexity.