Remove scanners from group and instance security dashboards

What does this MR do and why?

The scanners data is only used on the project security dashboard, but the query is still executed on the group and instance security dashboards. The query performs very poorly for groups because it has to traverse the group hierarchy and find all unique scanners (potentially hundreds of thousands). In some cases, the can prevent the vulnerability report from loading. Remove this query since it is expensive and not needed.

Fixes: #422143 (closed)

Screenshots or screen recordings

Group vulnerability report (tool filter still works):

Instance vulnerability report (tool filter still works):

MR acceptance checklist

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