Remove the subscription advertisement when you have more than 4 email accounts

[CodeMouse92 (Jason C. McDonald)]

I went ahead and started this, as it was mostly monotonous-type work.

Update/Drop Subscription Advert on 4+ Accounts

Remove all mention of old four account limit.

It would be trivial to remove the advert altogether. The question is just whether that would negatively impact Mailspring Pro subscriptions or not, as @bengotow pointed out.