Remove SafeMath from OUSD

[DanielVF]

Safe math operations are now built into the solidity compiler. We could remove the use of safe math from OUSD and save gas.

[DanielVF]

These have now been removed from the vault, but remain in OUSD.

[naddison36]

FYI, I've removed SafeMath from the OETH Vault as part of the collateral swap changes.

Note VaultAdmin still requires StableMath.