Remove scss variables, part 2

Removes:

• $gl-text-green
• $gl-text-green-hover
• $gl-text-red
• $placeholder-text-color
• $gl-link-color
• $gl-text-orange
• $gl-link-hover-color
• $gl-gray
• $gl-header-nav-hover-color

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/49807