UX: Fix composer helper z-index

Followup to #1064

That commits adds a higher z-index due to core changes, we no longer need
an iPad-specific z-index.

Author: pmusaraj

assets/stylesheets/modules/ai-helper/common/ai-helper.scss

@@ -704,10 +704,6 @@
   z-index: z("mobile-composer");
 }
 
-html.footer-nav-ipad .fk-d-menu[data-identifier="ai-composer-helper-menu"] {
-  z-index: z("ipad-header-nav") + 1;
-}
-
 .fk-d-toasts:has(.ai-proofread-error-toast) {
   top: unset;
   bottom: calc(var(--composer-height) - 5%);