enhance: remove box-shadow from kbd elements (#39943)

Remove the inset bottom shadow from `<kbd>` elements in markup and on
keyboard shortcut hints. The skeuomorphic key cap look is not needed,
the border alone marks the element as a key.

web_src/css/modules/shortcut.css

@@ -14,7 +14,6 @@ kbd {
   background-color: var(--color-box-body);
   border: 1px solid var(--color-secondary);
   border-radius: var(--border-radius);
-  box-shadow: var(--box-shadow-kbd);
 }
 
 .global-shortcut-wrapper > kbd {