[SCSS Variables Cleanup] Remove reassigned SCSS variables

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20841#note_90634650:

On that note though, I wish we didn't have so many variables assigned to the same color. I understand the thought behind it- if we want to update all the link colors at once we can just update $gl-link-color. But I'm not confident that all the links are actually set to that variable; there's a good chance they could be set to any of the variables with that same value.

Also- we have several places where variables are assigned to another variable, and then only used once. It makes the variables.scss file unnecessarily long.

I think it would be easier to find and track colors if each was only used once in variables.scss, with no reassigning. So, for example, if someone wants to make an element #b8d6f4, then they will use the variable $blue-200, and not $row-hover-border or $callout-info-border.

The only exception to this would be overriding default bootstrap variables (currently at the bottom of variables.scss, which should probably go into their own file.