Remove btn-sm-side-margin misc variable in favor of gitlab ui

What does this MR do and why?

Remove btn-sm-side-margin misc variable in favor of gitlab ui

The goal of this MR was to remove a custom variable that doesn't use spacing defined in Pajamas (7px) in favor of a variable from GitLab UI that maps to our design system (8px). Its a very small change, but we need to remove custom CSS that is not needed in favor of Pajamas/GitLab UI. The idea is that if the variable is later updated, this should cascade down rather than having to update all the individual variables in the GitLab repo.

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