Revert "feat(user): add ecto migration for user"

This reverts commit 9613fcebde.

priv/repo/migrations/20210809085801_create_users.exs

@@ -1,12 +0,0 @@
-defmodule Ketbin.Repo.Migrations.CreateUsers do
-  use Ecto.Migration
-
-  def change do
-    create table(:users) do
-      add :firebase_id, :string
-
-      timestamps()
-    end
-
-  end
-end