Remove `namespaces.plan_id` column

From https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6549#note_164327382, this column is unused but it causes confusion for devs (as seen in !18329 (comment 229283700)).

It also led to a bug because GroupPlansPreloader was depending on plan_id.

There are some code that uses this column and these should be investigated / removed:

1. GroupPlansPreloader #195432 (closed)
2. EE::User#has_paid_namespace? / EE::User#any_namespace_with_gold? !29014 (merged)
3. EE::Namespace.with_plan
4. EE::Project.for_plan_name !29019 (merged)
5. EE::Namespace#generate_subscription !28453 (merged)

cc @rdavila @smcgivern @jprovaznik