chore: remove stale FIXME about referer attribute typo (#14952)

## Summary
`Api::V1::Widget::BaseController#conversation_params` had a `FIXME: typo
referrer in additional attributes, will probably require a migration`
comment. On investigation, `additional_attributes[:referer]` is used
consistently across the whole codebase — automation rules,
conversation/contact filters, `ConversationInfo.vue`, and the widget
SDK's `referer_url` param — matching the historical (if misspelled) HTTP
`Referer` header convention. There is no actual inconsistency to fix,
and renaming the key would require a data migration and would break
existing customer automations built on the `referer` key, for no
functional benefit.

## What changed
- Removed the misleading FIXME comment. No behavior change.

app/controllers/api/v1/widget/base_controller.rb

@@ -29,7 +29,6 @@ class Api::V1::Widget::BaseController < ApplicationController
   end
 
   def conversation_params
-    # FIXME: typo referrer in additional attributes, will probably require a migration.
     {
       account_id: inbox.account_id,
       inbox_id: inbox.id,