fix(ui): NavTab font size (#30035)

NavTabs should consistently use the default font size (16px). Removing the hardcoded size of 15px from shared-components.less

static/app/components/layouts/thirds.tsx

@@ -111,7 +111,6 @@ export const HeaderNavTabs = styled(NavTabs)`
   }
   & > li > a {
     padding: ${space(1)} 0;
-    font-size: ${p => p.theme.fontSizeLarge};
     margin-bottom: 4px;
   }
   & > li.active > a {

static/less/shared-components.less

@@ -1168,7 +1168,6 @@ header + .alert {
     }
 
     > a {
-      font-size: 15px;
       padding: 0 0 10px;
       margin: 0;
       border: 0;