This requires a few things to be true:

1. Database connections are opened during routes. This is true for the
   GitLab application. Note this is not allowed anymore in Rails 7.
2. Multiple databases are configured. With a single database, Rails has
   always accidentally closed the database connection for the main
   database.

This fix removes prior attempts to drop the connection(s)

Database connections opened in other initializers are still an issue but
Rails kindly closes them for us. Unfortunately Rails has an initializer,
set_routes_reloader_hook which is run _after_ `config.after_initialize`.
The sequence goes like this:

1. Various initializers run, including route loading.
1. `config.after_initialize` blocks run (called by
   Gitlab::Application.finisher_hook). One of the
   `config.after_initialize` blocks run is to close all database
   connections.
1. `Gitlab::Application.set_routes_reloader_hook` initializer runs. This
   loads routes again.

So this fix is simple: we simply call the same thing Rails to close all
database connections again.