Remove Required Pipeline Configuration docs and drop column

Problem to solve

In 17.0 Required Pipeline Configuration was removed.

The docs still need to be removed and remove the ignore rule

Proposal

Remove the docs https://docs.gitlab.com/ee/administration/settings/continuous_integration.html#required-pipeline-configuration

Remove the ignore rule https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-the-column-release-m1