Clean up unused function left in the database.

db/migrate/20180308071922_drop_raise_read_only_function.rb

@@ -0,0 +1,11 @@
+class DropRaiseReadOnlyFunction < ActiveRecord::Migration[5.1]
+  def up
+    ActiveRecord::Base.exec_sql(
+      "DROP FUNCTION IF EXISTS raise_read_only CASCADE;"
+    )
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end