Remove deprecated `ERC20FailedDecreaseAllowance` error

[trevorgjohnson]

With the inclusion of #4585, increaseAllowance and decreaseAllowance was removed.

This just leaves 1 custom error that is related that now removed functionality.

[frangio]

Thanks this is correct. Do you want to submit a PR?

[trevorgjohnson]

Thanks this is correct. Do you want to submit a PR?

Yep! This issue should be resolved with #4604