This commit adds `ActiveJob.deprecator` and replaces all usages of
`ActiveSupport::Deprecation.warn` in `activejob/lib` with
`ActiveJob.deprecator`.

Additionally, this commit adds `ActiveJob.deprecator` to
`Rails.application.deprecators` so that it can be configured via
settings such as `config.active_support.report_deprecations`.

This commit also removes a defunct `ActiveSupport::Deprecation.silence`
call that was added in 9eb4b4ed but not
removed when the deprecation was completed in
10bd5e59.