Remove glql_work_items feature flag (always on)

What does this MR do and why?

Remove glql_work_items feature flag (always on)

For now we hard-code the GLQL FF as on, until it's similarly removed in GLQL itself. At that stage we can remove mentions of it entirely.

Changelog: other per https://docs.gitlab.com/development/feature_flags/#changelog (Flag is currently 'default: on' → Remove flag → Keep new code?).

References

• `glql_work_items` -- Allows using the work item... (#525480)
• Clean up GLQL-related feature flags (#591123)
• Remove glql_work_items feature flag (always on) (glql!336 - merged)
  • This is the MR which removes it from GLQL. It's not released yet (so we can't just stop passing glql_work_items to it yet); first we'll need to get Update dependency @gitlab/query-language-rust t... (!223628 - merged) over the line, deployed and confirmed; then we (or sectionanalytics probably) can make a new GLQL release with that change in it, and then update the monolith to use it. That update can remove the hard-coded FF.

How to set up and validate locally

1. Check out branch, remove the glql_work_items FF if it's enabled in your GDK manually.

2. Check that embedded views still work.

3. Test the following embedded view:

   ```glql
   query: epic = ()
   ```

4. It should fail with the following message:

   

This syntax was admitted before glql_work_items, but not as of it. If it compiles successfully, it indicates the FF has not "landed".

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