Update pipeline graph style to match design spec

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/54331

Gets rid of unnecessary styling in the pipeline graph, brings the graph closer to matching up with the design spec.

before	after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers