Drop a tweak on border-left in global nav

On !497 (merged), which was merged on 2020-07-28, level-0 in the global-nav-link always had border-left: 4px, which required us to the tweak this MR will remove, but it has been removed until now (while I spent much time for finding that MR, I could not make it 😭 ).

Screenshots

The following is for level-1, but similar to ones for level-0 and level-2:

before	after

Description

Changes in !503 (merged) might make this frontend ~bug implicit.

Closes #824 (closed)