help: Remove "Status and availability" mobile tabs.

The ability to view, set, edit a user's status, or to toggle invisible
mode are not yet implemented for the initial launch of the Flutter
mobile app, so we remove the legacy React Native app instructions.

Deletes static/images/help/mobile-x-icon.svg as it is no
longer used.

This feature is tracked in:
https://github.com/zulip/zulip-flutter/issues/197
https://github.com/zulip/zulip-flutter/issues/198
https://github.com/zulip/zulip-flutter/issues/1578

Part of #34748.

static/images/help/mobile-x-icon.svg

@@ -1 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="#6492fd" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather feather-x"><line x1="18" y1="6" x2="6" y2="18"></line><line x1="6" y1="6" x2="18" y2="18"></line></svg>