Pipeline Graph: Only apply top border to main graph

What does this MR do?

Updates CSS to remove a rogue border.

Screenshots or Screencasts (strongly suggested)

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Conformity

• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.