Remove obsolete migration config (#2769)

The compilation warning advises to use `Application.compile_env/3`
however there is no `:url` environment setting under `:plausible` key
anyway, so we might as well get rid of it completely.

priv/repo/migrations/20190618165016_add_public_sites.exs

@@ -1,12 +1,9 @@
 defmodule Plausible.Repo.Migrations.AddPublicSites do
   use Ecto.Migration
-  @host Application.get_env(:plausible, :url, :host)
 
   def change do
     alter table(:sites) do
       add :public, :boolean, null: false, default: false
     end
-
-    execute "update sites set public=true where domain='#{@host}'"
   end
 end