help: Remove "Unsubscribe from a channel" mobile tabs.

The ability to unsubscribe from a channel 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-check-icon.svg as it is no
longer used.

This feature is tracked in:
https://github.com/zulip/zulip-flutter/issues/1224

Part of #34748.

help/unsubscribe-from-a-channel.md

@@ -10,14 +10,6 @@ You can always unsubscribe from any channel in Zulip.
 
 1. Click **Unsubscribe**.
 
-{tab|mobile}
-
-{!channel-long-press-menu.md!}
-
-1. Tap **Unsubscribe**.
-
-{!channel-long-press-menu-tip.md!}
-
 {end_tabs}
 
 ## Alternate methods to unsubscribe from a channel
@@ -34,19 +26,6 @@ You can always unsubscribe from any channel in Zulip.
    (<img src="/static/images/help/desktop-web-check-icon.svg" alt="checkmark" class="help-center-icon"/>)
    icon to the left of a channel to unsubscribe from it.
 
-{tab|mobile}
-
-{!mobile-all-channels-view.md!}
-
-1. Tap the **checkmark**
-   (<img src="/static/images/help/mobile-check-icon.svg" alt="checkmark" class="help-center-icon"/>)
-   icon to the right of a channel to unsubscribe from it.
-
-!!! tip ""
-
-    You can also press and hold a channel until the long-press menu appears, and
-    tap **Unsubscribe**.
-
 {end_tabs}
 
 ### Via channel settings
@@ -66,16 +45,6 @@ You can always unsubscribe from any channel in Zulip.
     You can also use <kbd>Shift</kbd> + <kbd>S</kbd> to unsubscribe from the
     selected channel.
 
-{tab|mobile}
-
-{!channel-long-press-menu.md!}
-
-1. Tap **Channel settings**.
-
-1. Tap **Unsubscribe**.
-
-{!mobile-channel-settings-menu-tip.md!}
-
 {end_tabs}
 
 ## Related articles

static/images/help/mobile-check-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-check"><polyline points="20 6 9 17 4 12"></polyline></svg>