investigate whether we can remove ceiling() or currentCeiling() in the nav feed

[hieronx]

I am wondering if we actually need both methods:

 function ceiling(uint loan) public view returns (uint) {
        if (borrowed[loan] > currentCeiling(loan)) {
            return 0;
        }
        return safeSub(currentCeiling(loan), borrowed[loan]);
    }

function currentCeiling(uint loan) public view returns(uint) {
        bytes32 nftID_ = nftID(loan);
        return rmul(nftValues[nftID_], ceilingRatio[risk[nftID_]]);
    }

We only use ceiling here it seems. We could just use currentCeiling and remove the ceiling function.

Originally posted by @manuelpolzhofer in #567 (comment)