Clean-up pending-members alert in usage seats

What does this MR do and why?

We're removing an alert inside usage quotas > seats that points to the pending_members page (when saas_user_caps feature flag is on).

The pending members page can be accessed from a better location: Group > Members (added in !141584 (merged)).

Note: we're not adding a changelog because the changes are behind a feature flag.

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Don't checkout the branch yet
2. Enable saas_user_caps feature flag from rails console. (rails console then Feature.enable(:saas_user_caps))
3. With a group with an Ultimate plan
4. Navigate to Settings -> Permissions, LFS, 2FA -> User Cap
5. Set the user cap limit to 1
6. Navigate to the group members
7. Add one or more members (until you are above the user cap limit set in previous step)
8. Navigate to Usage Quota from the left sidebar under Settings
9. You should see an alert that you have pending members
10. Clicking the button in the alert should navigate to a pending members page
11. Checkout the branch and refresh the usage quotas > seats page
12. The banner should not appear

Related to #431187 (closed)