Fix remaining dust on oToken when burning all

[DanielVF]

Makes OUSD/OETH not leave dust when the entire balance of an account is redeemed. Closes #1495

To detect when we were burning the entire funds for an account, the old code did this:

currentCredits == creditAmount || currentCredits - 1 == creditAmount

This would not detect the problem if the creditAmount rounding error differed by more than 1.

Changed the code to the much more direct:

amount == balanceOf(account)

Maybe 1K gas more, but we only burn on redeems (super rare), and some AMO allocations.

Also:

• Added some helpful comments around the rounding behavior in the function
• Removed safe math from this function

Contract change checklist:

• Code reviewed by 2 reviewers.
• Copy & paste code review security checklist below this checklist.
• Unit tests pass
• Slither tests pass with no warning
• Echidna tests pass if PR includes changes to OUSD contract (not automated, run manually on local)

[DanielVF]

Note that I've got an another, possibly better fix version coming.

No need to review this one.

[DanielVF]

I'm just going to close this since I like #1680 so much better.