fixed migration

007_mod_plan_tasks_descr.rb doesn't roll back, because table name is missed:

MacBook-Pro-Admin-2:red-tower user$ rake redmine:plugins:migrate NAME=planner VERSION=0
Migrating planner (Planner)...
==  ModPlanTasksDescr: reverting ==============================================
-- remove_column(:created_at)
rake aborted!
An error has occurred, all later migrations canceled:

You must specify at least one column name. Example: remove_column(:people, :first_name)