Remove `feature_flags_legacy_read_only` feature flag

What does this MR do?

feature_flags_legacy_read_only feature flag is enabled on gitlab.com globally for a while. As it's confirmed as deemed stable, this MR removes the flag.

feature_flags_legacy_read_only_override was introduced for disabling the feature on the specific project with enabling it globally. Since it's not used at all today, we can remove it as well.

Close #240985 (closed)

Screenshots (strongly suggested)

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