Remove `increaseAllowance` and `decreaseAllowance`

[ControlCplusControlV]

Reasoning is pretty well described in this issue and really only add attack surface + phishing possibilities. (Also doesn't seem like your test suite uses it all except for tests on the main stablecoin?)

I know this is completely unsolicited though so I also understand if you close and move on, but felt bad asking to remove without making a PR