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.
We were using a <label> tag with no associated field where a <legend>
tag was more appropriate. With a fieldset, we also make it obvious these
fields are related.
