Drop broken "@ symbol as icon" setting

It was merged into pleroma-fe on 2022-02-03 in
76547fe66d and imported
into akkoma-fe on 2022-06-08 with the merge commit
f6cf509a04.

However, something went wrong in the merge and while the setting
and its infrastructure exist, it is never used anywhere and @ is
always displayed as text.

Given it existed in this broken state for nearly one and a half years,
never worked on akkoma-fe and no bugs were filed about this, it appears
nobody cares, so let’s just remove it.

src/components/settings_modal/tabs/general_tab.vue

@@ -483,14 +483,6 @@
             </BooleanSetting>
           </li>
         </ul>
-        <li>
-          <BooleanSetting
-            path="useAtIcon"
-            expert="1"
-          >
-            {{ $t('settings.use_at_icon') }}
-          </BooleanSetting>
-        </li>
         <li>
           <BooleanSetting path="mentionLinkShowAvatar">
             {{ $t('settings.mention_link_show_avatar') }}