The interpolation of the rem-calc function made the code harder to read.
Besides, for new code we've already agreed upon using rem units instead
of `rem-calc`.
For instance, we had the following code:
```
width: calc(100% - #{2 * rem-calc(10)});
```
Now, `2 * rem-calc(10)` is the same as `rem-calc(20)`, but we were using
`2 *` to make it clear that this value was related to the value of the
`margin-left` property, which was `rem-calc(10)`.
IMHO using `0.625rem` for the margin and `2 * 0.625rem` for the width
the code is easier to read because there are no interpolation and no
complex operations involved.
This way we remove duplication and it'll be easier to add better support
for RTL languages.
In a few years this might not be necessary since support for the `gap`
property in a flexbox layout will improve. At the time of writing,
however, only 86.5% of the browsers support it [1].
[1] https://caniuse.com/flexbox-gap
