Drop "pool" column from usage_by_user_tab.vue

What does this MR do and why?

Drop "pool" column from usage_by_user_tab.vue.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/578333+

Related to #577702

Screenshots or screen recordings

Before	After

How to set up and validate locally

• yarn storybook:start
• Navigate to http://localhost:6006/?path=/story/ee-usage-quotas-usage-billing-app--default

MR acceptance checklist

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