Removing user-defined units

Is there a way to remove user-defined units? I create units for currency conversion using:

```javascript
math.createUnit('USD', {aliases: ['usd']})

var rates = currencyRates //Rates in json format
Object.keys(rates).map((currency) => {
math.createUnit(rates[currency].code, {
definition: math.unit(rates[currency].inverseRate + 'USD'),
aliases: [rates[currency].code.toLowerCase()]
}, {
override: true
})
})
```

But if I no longer want to use currencies, is there a way to remove them without creating a new mathjs instance (in my case without refreshing browser window)? If not, may I suggest introducing a math.removeUnit() method?