Remove feature flag behind new projects archive settings UI

What does this MR do and why?

Currently, the project archive settings behaves as:

1. When archive_group flag is disabled, the user can freely archive/unarchive the project.
2. When archive_group flag is enabled, the user cannot unarchive the project if it belongs to an archived group.

Based on our decision in #561661 (comment 2711591938), we will only cover the group archiving feature behind the flag. Group unarchiving will always be available.

This means that there is no need to keep the behavior (1) such that regardless of the feature flag state:

1. Users cannot archive/unarchive projects that belongs to an archived group.
2. Users must unarchive the group first in the settings page.

References

Closes #566483 (closed)

Screenshots or screen recordings

When a project belongs in an archive group AND the archive_group flag is disabled:

Before	After

How to set up and validate locally

1. Enable archive_group feature flag.
2. Go to Group > Settings > Advance > Archive group > Archive
3. Go to Project > Settings > Advance > Archive/Unarchive project

MR acceptance checklist

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