This reverts commit e00fb2bdc2090e9cabeb1eb35a2672a882cc96e9.

# Conflicts:
#	.rubocop.yml
#	.rubocop_todo.yml
#	lib/gitlab/ci/config/entry/global.rb
#	lib/gitlab/ci/config/entry/jobs.rb
#	spec/lib/gitlab/ci/config/entry/factory_spec.rb
#	spec/lib/gitlab/ci/config/entry/global_spec.rb
#	spec/lib/gitlab/ci/config/entry/job_spec.rb
#	spec/lib/gitlab/ci/status/build/factory_spec.rb
#	spec/lib/gitlab/incoming_email_spec.rb

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.

We assume that when someone adds a key for the configuration entry, but
does not provide a valid value, which causes entry to be `nil`, then
entry should be considered as the undefined one. We also assume this is
semantically correct, this is also backwards compatible with legacy CI
config processor.

See issue #18775 for more details.

This makes use of `ActiveModel::Validations` encapsulated in a separate
class that is accessible from a node object.

This reverts commit 13e37a3e.