Remove references to OmniAuth UltraAuth integration

Summary

The OmniAuth UltraAuth strategy laid out in gitlab-foss!26348 (merged) is no longer active ref gitlab-foss!26348 (comment 313393614)

Steps to reproduce

Our docs refer to UltraAuth. That URL no longer works, and per gitlab-foss!26348 (comment 313393614), that service has been "shut down"

Example Project

(If possible, please create an example project here on GitLab.com that exhibits the problematic behavior, and link to it here in the bug report)

(If you are using an older version of GitLab, this will also determine whether the bug is fixed in a more recent version)

What is the current bug behavior?

UltraAuth integration no longer exists

What is the expected correct behavior?

Since the UltraAuth app has been shut down, the correct behavior would be to remove all references to UltraAuth in our docs and code.

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's tough to read otherwise.)

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)