UX: remove old menu border hack (#31344)

We used to hide the bottom border of the header buttons when menu panels
were open (so they'd appear connected), but this is no longer
necessary... so all this vestigial CSS does is sometimes create a stray
line like this:


![image](https://github.com/user-attachments/assets/12357bf3-2bc7-4604-8b76-7e54460a4175)

app/assets/stylesheets/common/base/header.scss

@@ -197,22 +197,6 @@
       > .d-icon {
         color: var(--primary-medium);
       }
-
-      &::after {
-        display: block;
-        position: absolute;
-        top: 100%;
-        left: 0;
-        z-index: z("header") + 1; // Higher than .menu-panel
-        width: 100%;
-        height: 0;
-        content: "";
-        border-top: 1px solid var(--secondary);
-      }
-
-      &:hover {
-        border-bottom: none;
-      }
     }
   }