Remove redundant toast.scss file and variables

What does this MR do?

In 20356 we noticed that the color of the close button on the toast component is incorrect.

After further investigation, I noticed that the toast.scss stylesheet is still present within gitlab even though it was migrated to gitlab-ui in gitlab-ui!733 (merged) which conflicted with the stylesheet from gitlab-ui.

This MR removes the redundant styles from gitlab.

Screenshots

before	after

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides