Remove work_items_project_issues_list FF

What does this MR do and why?

• This merge requests removes the work_items_project_issues_list feature flag, which has been enabled by default in !204139 (merged)
  • The FF has also been enabled in production for over 1 week: #556695 (comment 2743801563)
• Additionally, one of the spec failures is related to a regression on when we show the New item button for logged out users. The changes in this MR revert this to the legacy behaviour, which shows the button for logged out users, and redirects them to the login page when selected

References

• #569876 (closed)
• #556695 (closed)
• &17646 (closed)

Screenshots or screen recordings

Not provided

How to set up and validate locally

1. Navigate to the project issues page for a given project which has some issues
2. Observe that the new project issues list loads (e.g. by looking at the performance bar and noting the requests made to getWorkItemsSlimEE / getWorkItemsFullEE)

Additionally, to test the behaviour of the New issue button change also included in this MR:

1. As a logged out user, navigate to the project issues page
2. Observe the New issue button is displayed
3. When clicked, this should redirect the logged out user to the login page
4. Repeat the steps above on the Epics page for a group. Observe that logged out users do not have the "New item" button available

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #569876 (closed)