Remove waiting_time from runner model

After removing the logic that stores the duration that has been
waited for a runner in the runner, this now also removes the column
from the database as it is not used anymore.

db/migrate/20210611101330_remove_waiting_time_from_runners.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class RemoveWaitingTimeFromRunners < ActiveRecord::Migration[6.1]
+  def change
+    remove_column :runners, :waiting_time
+  end
+end