Fix go-get support for relative urls

What does this MR do and why?

Contributes to #508593 (closed)

Problem

!161162 (merged) changed the generation logic for urls.

Before we used to build URLs based on Gitlab.config.gitlab.url value

After the change, we use Gitlab.config.gitlab.host value.

The problem with this approach that it ignores relative URLs configuration.

Solution

Revert changes to rely on Gitlab.config.gitlab.url as before.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Relative URL (external_url) is not respected wi... (#508593 - closed)

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Before	After

How to set up and validate locally

1. Define relative_url_root in gdk.yml

relative_url_root: '/gitlab'

2. gdk reconfigure

3. gdk restart

4. Visit http://gdk.test:3000/gitlab/gitlab-org/gitlab-test?go-get=1

5. You should see that relative url /gitlab is missing in the url

6. Enable feature flag: Feature.enable(:go_get_handle_relative_url)

7. gdk restart

8. Visit http://gdk.test:3000/gitlab/gitlab-org/gitlab-test?go-get=1

9. The relative url /gitlab will be present in the url

After testing

1. Remove relative_url_root from gdk.yml
2. gdk reconfigure to restore the previous state

Numbered steps to set up and validate the change are strongly suggested.