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/modules/instance.js

@@ -21,7 +21,6 @@ const defaultState = {
   background: '/static/aurora_borealis.jpg',
   collapseMessageWithSubject: true,
   greentext: false,
-  useAtIcon: false,
   mentionLinkDisplay: 'short',
   mentionLinkShowTooltip: true,
   mentionLinkShowAvatar: false,