Remove "string" column from "ppid" table

This was added by accident in 4e18f384.

closes #7598

db/migrate/20170827231800_remove_string_column_from_ppid.rb

+# frozen_string_literal: true
+
+class RemoveStringColumnFromPpid < ActiveRecord::Migration[5.1]
+  def change
+    remove_column :ppid, :string, :string, limit: 32
+  end
+end