Remove gitlab_rails['db_pool'] setting

This is part of gitlab-com/gl-infra/scalability#495 (closed)

As explained in that MR, we have come to realize that the pool: setting in database.yml is not useful and we should just let the application manage the database connection pool size. This is because the connection pool size should depend on the number of application threads. If an administrator wants to see fewer database connections, they should reduce the number of Ruby threads and/or processes.