minor css fix (#26509)

* minor css fix

* Update _search.scss

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/sass/components/_search.scss

@@ -211,6 +211,7 @@
         }
 
         .search-bar {
+            box-shadow: none;
             color: rgb(var(--center-channel-color-rgb));
         }
     }