help: Update "Inbox" instructions for Flutter launch.

Also, updates the options in 'Filter by topic status'
for the web app as "Unmuted topics" has been changed
to "Standard view".

Deletes static/images/help/mobile-expand-more.svg,
static/images/help/mobile-expand-less.svg, and
static/images/help/mobile-inbox-icon.svg as they are
no longer used.

Part of #34748.

static/images/help/mobile-expand-less-icon.svg

@@ -1,4 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="#000000">
-    <path d="M0 0h24v24H0z" fill="none"/>
-    <path d="M12 8l-6 6 1.41 1.41L12 10.83l4.59 4.58L18 14z"/>
-</svg>

static/images/help/mobile-expand-more-icon.svg

@@ -1,4 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="#000000">
-    <path d="M0 0h24v24H0z" fill="none"/>
-    <path d="M16.59 8.59L12 13.17 7.41 8.59 6 10l6 6 6-6z"/>
-</svg>

static/images/help/mobile-inbox-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-inbox">
-    <polyline points="22 12 16 12 14 15 10 15 8 12 2 12"></polyline>
-    <path d="M5.45 5.11L2 12v6a2 2 0 0 0 2 2h16a2 2 0 0 0 2-2v-6l-3.45-6.89A2 2 0 0 0 16.76 4H7.24a2 2 0 0 0-1.79 1.11z"></path>
-</svg>