Enable merge trains project-level config by default

What does this MR do?

We're going to introduce pipeline parallelization of merge trains in https://gitlab.com/gitlab-org/gitlab-ee/issues/11222. This completes the set of functionality of merge train which we devised at first, thus we can remove the project-level config, which is default off today, to be on by default.

Even though this merge train is always on, users can still choose immediate merge as one of the merge strategies. Thus, this does not impact user's flow significantly.

For the safeness, merge train still can be disabled forcibly with the feature flag merge_trains_enabled. To disable, execute Feature.disable(:merge_trains_enabled).

Related https://gitlab.com/gitlab-org/gitlab-ee/issues/11222

With this iteration, we also remove the problem with merge trains being one-at-a-time, therefore we can remove the option to enable merge trains and have this strategy be the default

Also, see https://gitlab.com/gitlab-org/gitlab-ee/issues/11222#note_174302668.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team