#212848 Removed UltraAuth integration for OmniAuth

What does this MR do?

It removes UltraAuth integration for OmniAuth introduced in gitlab-foss!26348 (merged) and gitlab-foss!28941 (merged)

Resolves #212848 (closed)

Screenshots

database query plans

db/migrate/20200508021128_remove_ultraauth_provider_from_identities.rb

Query:

DELETE FROM identities WHERE provider = 'ultraauth';

Plan: https://explain.depesz.com/s/pJnl

Recommendations (internal):

❗ Query processes too much data to return a relatively small number of rows. – Reduce data cardinality as early as possible during the execution, using one or several of the following techniques: new indexes, partitioning, query rewriting, denormalization. See the visualization of the plan to understand which plan nodes are the main bottlenecks.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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