Allow saving deprecated `import_sources`

What does this MR do?

Allow saving deprecated import_sources.

Validation on google_code value in import_sources was enforced prior to DB cleanup was executed. This causes issues with saving application_settings on an instance, including GitLab.com, that still has google_code as a value in import_sources.

gitlabhq_production=> select import_sources from application_settings;
          import_sources          
----------------------------------
 ---                             +
 - github                        +
 - bitbucket                     +
 - bitbucket_server              +
 - gitlab                        +
 - google_code                   +
 - fogbugz                       +
 - git                           +
 - gitlab_project                +
 - gitea                         +
 - manifest                      +
 - gitlab_custom_project_template+

This change is an interim step before a proper DB migration is introduced to clean up the import_sources.

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