Closes #41467 

My first PR in Zed, any guidance or tips are appreciated.

This fixes the flickering/resize loop that occurred on the tab
immediately to the right of a pinned tab.

Removed the conditional border on the pinned tabs container. The border
was a visual indicator to show when unpinned tabs were scrolled, but it
wasn't essential and was causing the layout thrashing.

Release Notes:

- Fixed

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Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>