Fix -zf-each-breakpoint without small bp

`$breakpoint-classes` is a list, not a map.

Use `sl-remove()` from SassyList
(https://github.com/at-import/SassyLists) instead of `map-remove()` to
remove the small breakpoint from `$breakpoint-classes`.

Other changes:
- Rename `$map` to `$list`.
- Rename `$size` to `$name`. `$breakpoint-classes` contains breakpoint
names, not sizes. Keep `$-zf-size` for compatibility.