Breakpoints / Media Queries refactor

- Rename `$a-leftCol-width` into `$left-column`
- Rename `$a-leftColWide-width` into `$left-column-wide`
- Remove unused breakpoint `desktopAd`
- Remove unused variable `$unitHeight`
- Replace `gs-span(2)` and `gs-span(3)` in code by `$left-column` and `$left-column-wide` where applicable

I kept breakpoint names the same to avoid disturbing other devs. Plus, existing names have served us well so far.

/cc @phamann this should solve the issue in `_facia-container.scss` where we had a rogue breakpoint

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I agree with that, but that's not the point of this PR, if that's okay with you I'd like to go through this partial later (and I'd like to do some renaming as well because I think we could find a better file name for the _disc.scss partial)