Remove feature flag from upgrade link in profile menu [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Removes feature flag :upgrade_link_in_user_menu_a

Feature flag was added in 13.11, converted from an experiment feature flag :upgrade_link_in_user_menu_a_experiment_percentage that had previously been running at 100%: !56591 (merged), to be removed after one milestone in 13.12.

• only displays Upgrade link to users on bronze or silver plans.
• does not display to users if their personal namespace or any of their groups are already on the top tier gold plan.

Related to successful experiment cleanup issue: https://gitlab.com/gitlab-org/gitlab/-/issues/300824

Original experiment MR: !29428 (merged)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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