[Forwardport] Add default schedule config for sitemap_generate job

[rostyslav-hymon]

Original Pull Request

magento#14822

Description

There was no default schedule configuration for sitemap_generate job, and I think this is causing confusion for some people.

Also I have removed a redundant constant in the sitemap observer.

I think more work is required on sitemaps following this.

Fixed Issues (if relevant)

1. Magento 2.0.7 XML sitemap is not generated by schedule magento/magento2#5768: Magento 2.0.7 XML sitemap is not generated by schedule (potentially, waiting feedback from creator)

Manual testing scenarios

1. Create sitemap via admin
2. Do not sitemap generation schedule settings in Store > Configuration > Catalog > XML Sitemap > Generation Settings
3. Configure magento cron
4. Sitemap should be updated at midnight.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)