Remove or update services:doc rake task

Summary

The rake task can be found in lib/tasks/services.rake.

As we discovered in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31675#note_203489976, the output of this rake task is inconsistent with the formatting of the documentation. It is definitely no longer being used.

Improvements

The task should be either removed or updated. Additionally, if it gets updated, then its use should be enforced via CI to avoid future inconsistencies.

Risks

If the task gets updated, it might clash with another tool that might already be used to update this documentation (though I could not find one).

Involved components

The rake task services:doc

Optional: Intended side effects

Optional: Missing test coverage

The task currently has no tests. At the very least, if the task continues to be used, there should be a test case that runs the task and compares the diff to enforce the tasks usage and help keep our docs up to date.