[Feature flag] Remove `return_disabled_vulnerability_graphql_filters`

What does this MR do and why?

In a previous MR we started calculating which vulnerability-related filters are disabled due to group size limits.

The return_disabled_vulnerability_graphql_filters feature flag has been enabled in production for several days, and everything is behaving as expected.

This change cleans up that FF.

References

• Add `disabled_filters` to vulnerability graphql... (!180973 - merged) • Michael Becker • 17.10
• [Feature flag] Enable return_disabled_vulnerabi... (#520703 - closed) • Michael Becker • 17.10

MR acceptance checklist

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epic: &13340
related to: #517915 (comment 2399220371)
resolves: #520703 (closed)
MR: !184617 (merged)
Changelog: changed
EE: true