Remove activity border in work items

What does this MR do and why?

• Removes border between work item widgets and activity (#463598 (comment 2052619113))
• Remove margin to keep things consistent with issues/merge requests

Screenshots or screen recordings

Before	After

How to set up and validate locally

Check a work item (task, objective, epic etc)

Links

• ✂️ UX Paper Cuts 17.4 → Comments & system notes (#469616 - closed)