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.

spec/factories/runner.rb

@@ -3,9 +3,8 @@
 # This factory does not request the runner management as the id is already provided.
 FactoryBot.define do
   factory :runner do
-    sequence(:runner_id) {|n| "test-runner-id-#{n}" }
+    runner_id { SecureRandom.uuid }
     association :execution_environment, factory: :ruby
     association :user, factory: :external_user
-    waiting_time { 1.0 }
   end
 end