help: Update "View your mentions" instructions for Flutter launch.

Deletes static/images/help/mobile-at-sign-icon.svg as it's no
longer used.

Part of #34748.

help/include/view-mentions.md

@@ -16,9 +16,10 @@
 
 {tab|mobile}
 
-1. Tap the **Mentions**
-   (<img src="/static/images/help/mobile-at-sign-icon.svg" alt="at-sign" class="help-center-icon"/>)
-   tab at the top of the app.
+1. Tap the **Menu** (<i class="zulip-icon zulip-icon-mobile-menu mobile-help"></i>)
+   tab in the bottom right corner of the app.
+
+1. Tap <i class="zulip-icon zulip-icon-at-sign mobile-help"></i> **Mentions**.
 
 1. Browse your mentions. You can tap on a message recipient bar to go
    to the conversation where you were mentioned.

static/images/help/mobile-at-sign-icon.svg

@@ -1,4 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="#6492fd" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather feather-at-sign">
-    <circle cx="12" cy="12" r="4"></circle>
-    <path d="M16 8v5a3 3 0 0 0 6 0v-1a10 10 0 1 0-3.92 7.94"></path>
-</svg>