Find: <xf:if is="$thread.canWatch()">
Replace:
<xf:if is="{$thread.user_id} == {$xf.visitor.user_id} AND {{ $xf.visitor.hasPermission('calendar', 'addRemoveOwn') }}">
<xf:button href="{{ link('threads/calendar-menu', $thread) }}" class="button--link">
{{ phrase('calendar_calendar_menu') }}
</xf:button>
<xf:elseif is="{{ $xf.visitor.hasPermission('calendar', 'addRemoveAll') }}" />
<xf:button href="{{ link('threads/calendar-menu', $thread) }}" class="button--link">
{{ phrase('calendar_calendar_menu') }}
</xf:button>
</xf:if>
$0