changes in the remove_coupon view json response

@chrisndodge I have made changes in the remove coupons view. Kindly have a look into those changes
Thanks  
The `format()` function needs to be called *after* the `gettext()` function. The code should be:
```python
_('coupon with the coupon id ({coupon_id}) is already inactive').format(coupon_id=coupon_id)
```