The 'labels' parameter shouldn't affect custom ordering

Example:

```python
data = {
'g': ['b', 'a', 'a', 'a', 'c', 'c'],
}
labels = {'a': 'A', 'b': 'B', 'c': 'C'}

ggplot(data) + \
geom_bar(aes(x=as_discrete('g', order_by='..count..'))) + \
scale_x_discrete(labels=labels)
```

Output:

Expected output:

So, I expected ordering as in `as_discrete()`, but got ordering as in the dictionary. Moreover, my expectations coincide with the way it is done in ggplot2 (there you can make a similar example, but with `forcats::fct_infreq()` instead of `as_discrete()`).