Remove user count from Error Tracking list and details page

What does this MR do and why?

Remove user count from Error Tracking list and details page ( only for integrated error tracking )

Closes gitlab-org/opstrace/opstrace#2549 (closed)

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

As running the full integrated error tracking stack is not trivial, I'd suggest to just run GDK locally and mock responses with a Chrome Extension. I personally use Requestly. You can import the mock rules here: https://app.requestly.io/rules#sharedList/1704297812909-requestly_shared_list_03_01_2024

To switch between integrated and non-integrated error tracking, you need to

1. Flip the feature flag :integrated_error_tracking
2. Update the mock response and change integrated: true in the JSON