Remove environment dependency for db:schema:load

All of the behavior :environment was giving (that db:schema:load needed)
was provided as well with :load_config.

This will address an issue introduced in
https://github.com/rails/rails/pull/15394. The fact that db:schema:load
now drops and creates the database causes the Octopus gem to have [an
issue](https://github.com/tchandy/octopus/issues/273) during the drop
step for the test database (which wasn't happening in db:schema:load
before). The error looks like:

    ActiveRecord::StatementInvalid: PG::ObjectInUse: ERROR:  cannot drop the currently open database
    : DROP DATABASE IF EXISTS "app_test"

Because of the timing, this issue is present in master, 4-2-*, and
4.1.8.

A note to forlorn developers who might see this: "Additionally" in a
commit message means you should have a separate commit, with a separate
justification for changes. Small commits with big messages are your
friends.
Merge pull request #17920 from calebthompson/dont-rely-on-environment-task-for-schema-load

Remove environment dependency for db:schema:load