Remove block_code_suggestions_when_duo_disabled flag

What does this MR do and why?

Remove block_code_suggestions_when_duo_disabled flag

The flag was enabled on GitLab.com and the feature is stable, so remove the flag and keep the behavior on for everyone.

Code Suggestions is now always blocked when GitLab Duo is turned off for the namespace that governs the request. This is the behavior that the flag controlled.

Delete the flag definition, the Feature.enabled? check, and the tests that covered the flag being off.

References

Related to #607850 (closed)

Related to #606495 (closed)

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