Refactor gitlab-com membership methods

What does this MR do and why?

Describe in detail what your merge request does and why.

1. Remove ::Feature.enabled?(:gitlab_employee_badge) from gitlab_com_group_member_id? so that it can be used to check if a user is a team member for reasons other than displaying the employee badge.
2. Refactor usages of gitlab_com_group_member_id? inside the User class.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.