remove unused .tsmb-form--slash::after style

Follows-up ca5519d, which moved this from applying last,
in a media query, to hide the keyboard short cut on narrow viewports
(likely touchscreen devices); to instead apply before the actual style
unconditionally, thus never actually applying as it is immediately
overriden by another style right below it.

The default for `::after`, is already to not exist / be hidden.

Ref #2.

typesense-minibar.css

@@ -121,10 +121,6 @@ typesense-minibar form::before {
   display: block !important;
 }
 
-.tsmb-form--slash::after {
-  display: none;
-}
-
 .tsmb-form--slash:not(.tsmb-form--open):not(:focus-within)::after {
   content: '/';
   display: inline-block;