help: Remove "Start a video call" mobile tab.

The ability to add a link to a video call is 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-video-icon.svg as it is no
longer used.

This feature is tracked in:
https://github.com/zulip/zulip-flutter/issues/1000

Part of #34748.

help/start-a-call.md

@@ -31,28 +31,6 @@ administrators.
     You can replace the "Join video call." label for the link with any text you
     like.
 
-{tab|mobile}
-
-1. Navigate to a channel, topic, or direct message view.
-
-1. Tap the **Add video call**
-   ( <img src="/static/images/help/mobile-video-icon.svg" alt="video"
-   class="help-center-icon"/> )
-   icon at the bottom of the app. This will insert a **Click to join video call**
-   link into your message.
-
-1. If you are in a channel view, choose a destination topic by tapping the
-   compose box and selecting an existing topic or typing a new topic name.
-
-1. Send the message.
-
-1. Tap the link in the message to start or join the call.
-
-!!! tip ""
-
-    You can replace the "Click to join video call" label for the link with any
-    text you like.
-
 {end_tabs}
 
 ## Start a voice call

static/images/help/mobile-video-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-video">
-    <polygon points="23 7 16 12 23 17 23 7"></polygon>
-    <rect x="1" y="5" width="15" height="14" rx="2" ry="2"></rect>
-</svg>