Remove .issuable-row background in milestone's page_bundles

What does this MR do and why?

Remove .issuable-row background in milestone's page_bundles

The background is set to white on light theme, but some grayish color on dark theme. This change will remove the background from the item making it clearer and easier to read.

This is a follow-up to this thread: !164642 (diffs, comment 2136848416)

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Before	After

How to set up and validate locally

• Set dark theme in profile > preferences
• Navigate to a milestone with issues inside (example)
• Check the grayish background on the issues
• Checkout this branch
• Refresh the page and check the background of the issues