Remove `scim_identities` feature flag

What does this MR do?

Related to #212620 (closed) and #251080 (closed)

Removes the scim_identities feature flag. This addresses the need to update the feature flag details requested as part of #251080 (closed). Rather than mess with updating the flag we can safely get rid of it. It's a GitLab.com feature that has been enabled for some months now.

I didn't create a changelog entry because this was a feature flag that already defaulted to true and also only affects GitLab.com. Users don't really need to know about it.

Screenshots

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